Add the following folder structure to it:

wpdiscuz/
├── class.WpdiscuzWalker.php
└── layouts/
│   ├── 1/
│   │      ├── author.html
│   │      ├── city.html
│   │      ├── header.html
│   │      └── style.css  

finally I will summarize what I have done to archive this. It might inspire in similar situation.

Start by adding your additional Field in your Form

You might want to change the META-KEY under Advanced Options to something more readable to use in your code.

Have a look into the Database in Table `commentmeta` to see the real META-KEY because it is case-sensitive!

Add the following folder structure to it:

wpdiscuz/
├── class.WpdiscuzWalker.php
└── layouts
├── 1
│   ├── author.html
│   ├── city.html
│   ├── header.html
│   └── style.css

Create the file  city.html  is simple as:

<div class="{CITY_WRAPPER_CLASSES}">
    <span class="pre">{PRE_CITY}</span>&nbsp;{CITY}&nbsp;
</div>

<div class="{HEADER_WRAPPER_CLASSES}">
    {AUTHOR}
    {CITY}
    {DATE}
    {STATUS}
    {SHARE}
    <div class="wpd-space"></div>
    {LINK}
</div>

#wpdcom .wpd-comment-header .wpd-comment-city{ }
#wpdcom .wpd-comment-header .wpd-comment-city .pre{font-style: italic;}

@@ -93,6 +93,33 @@
             $commentWrapperClass[] = "wpd-reply";
         }
 
+	$showCity = false;
+	$city = isset($commentMetas) ? strval($commentMetas) : "";
+	if ($city) {
+	    $search[] = "{CITY_WRAPPER_CLASSES}";
+	    $search[] = "{PRE_CITY}";
+	    $search[] = "{CITY}";
+	    $replace[] = "wpd-comment-city";
+	    switch( get_locale() ) {
+	      case 'de_DE':
+	          $replace[] = "aus";
+	          break;
+	      case 'fr_FR':
+	      case 'es_ES':
+	          $replace[] = "de";
+	          break;
+	      case 'it_IT':
+	          $replace[] = "da";
+	          break;
+	      case 'pl_PL':
+	          $replace[] = "z";
+	          break;
+	      default:
+	          $replace[] = "from";
+	    }
+	    $replace[] = $city;
+	    $showCity = true;
+	}
+
         $showDate = false;
         if ($this->options->thread_layouts) {
 
@@ -429,7 +451,8 @@
             $replace[] = $content . $comment->comment_content;
             $replace[] = "wpd-comment-left " . esc_attr($commentLeftClass);
             $leftComponent = $showAvatar || $showLabel || $showFollow ? str_replace(, [$showAvatar ? $args : "", ($showLabel ? $args : "") . apply_filters("wpdiscuz_after_label", "", $comment), $showFollow ? $args : ""], $args) : "";
-            $headerComponent = str_replace(, [$args, $showDate ? $args : "", $showStatus ? $args : "", $showShare ? $args : "", $showLink ? $args : ""], $args);
+//            $headerComponent = str_replace(, [$args, $showDate ? $args : "", $showStatus ? $args : "", $showShare ? $args : "", $showLink ? $args : ""], $args);
+            $headerComponent = str_replace(, [$args, $showCity ? $args : "", $showDate ? $args : "", $showStatus ? $args : "", $showShare ? $args : "", $showLink ? $args : ""], $args);
             $footerComponent = $showVote || $showReply || $afterReplyButton || $showTools || $showToggle ? str_replace(, [$showVote ? $args : "", ($showReply ? $args : "") . $afterReplyButton, $showTools ? $args : "", $showToggle ? $args : ""], $args) : "";
             $rightComponent = str_replace(, [$headerComponent, $showReplyTo ? $args : "", $args . $lastEdited, $footerComponent], $args);
             $wrapperComponent = str_replace(, , $args);

I will post a final message recompiling all my changes so others my implement the same or similar solutions.

Thanks for your help! 😍 

In our case the solution works fine. 

Could you please send the admin login details to infogvectors.com email address? I'll ask the developers to check the issue. 

unfortunately this doesn't work as expected. I changed the META_KEY to 'CITY' and also applied it to older entries.

$city = isset($commentMetas) ? $commentMetas : "";

but it seems the Key is not in the List. I failed to `print_r` or `var_dump` the variable so I can't say for sure.

In the Database in Table `commentmeta` I see entries like

 It doesn't matter if I write the `meta_key` in upper or lower case.

The update safe way for this kind of customization is described here: https://wpdiscuz.com/docs/wpdiscuz-7/customization/custom-template-and-style/

I may suggest you to get the value of the city field using the $commentMetas variable in the class.WpdiscuzWalker.php file.

The meta_key you can find by editing the current field, click on the "Advanced Options" button.  

Here is an example: 

isset($commentMetas) ? $commentMetas : ""

You'll need to change the red marked part with corresponding meta_key. 

-            $headerComponent = str_replace(, [$args, $showDate ? $args : "", $showStatus ? $args : "", $showShare ? $args : "", $showLink ? $args : ""], $args);
+            $headerComponent = str_replace(, [$args, $showCity ? $args : "", $showDate ? $args : "", $showStatus ? $args : "", $showShare ? $args : "", $showLink ? $args : ""], $args);

the related variable (is) should be evaluated like

+
+        $showCity = false;
+        //$city .= apply_filters("wpdiscuz_comment_city", "", $comment, $user, $args);
+        //$city = isset($commentMetas) ? strval($commentMetas);
+        $city .= 'London';
+        if ($city) {
+            $search[] = "{CITY_WRAPPER_CLASSES}";
+            $search[] = "{PRE_CITY}";
+            $search[] = "{CITY}";
+            $replace[] = "wpd-comment-city";
+            $replace[] = "from";  //TODO: language depended 'from'
+            $replace[] = $city;
+            $showCity = true;
+        }
+

The city.html is simple as:

<div class="{CITY_WRAPPER_CLASSES}">
<span class="pre">{PRE_CITY}</span>&nbsp;{CITY}&nbsp;
</div>

My Problem now is that I did not manage to load the META-VAR {CITY} - which is saved with the comment - from the database. Both variants (see above) won't work.
How do I get the value?

...and how can I make my changes persistent without loosing them on Update/Upgrade of the Plugin? Simply copy the changes to the 'plugins' folder of my Child-Theme?

Thanks for any constructive help!

Sorry for the late response. 

We're really sorry, but there is not any simple solution we can post here. We may suggest you use the wpdiscuz_comment_author hook for this customization.

Mayby you, @asti, might help me with this?