Dear Freddy!

[...] In our forum, users are continuing their topics and discussions sometimes as a new topic, which drives me slightly mad.

Full AKN...

Your forum is of the 1.x branch, so I will answer accordingly.

Is there an option or workaround how to merge related / same topics?

Just a workaround (if you have access to WebMySQL on your server) - and an example:

|-post 2
  |-reply 2.1
|-post 1
  |-reply 1.1
    |-reply 1.1.1
    |-reply 1.1.2
  |-reply 1.2

Post 2 (and the reply 2.1) was opened as a new thread and you want to link it to reply 1.1.1...
If you are not familiar with the WebMySQL-interface you should backup the database first in order to avoid nasty surprises...

• Navigate to post 2 and note the post's ID (in the address line of the browser following ...?id=). Do the same for reply 2.1, post 1, and for reply 1.1.1
  
• Fire up WebMySQL
  
• Open the forum's database and navigate to the table 'forum_entries'
  
• Open 'forum_entries' click 'Browse': you see that every post has three IDs, namely id (the post's individual ID), pid (the ID of the 'parent' of the post), and tid (the ID of the thread)
  
• Following rules apply:
  • first post in a thread: id=tid, pid=0
    
  • first reply to a post: pid=tid
• Navigate to reply 1.1.1 and note the tid
  
• Navigate to post 2 and change the tid to the id of post 1; change the pid to the id of reply 1.1.1.
  
• Navigate to reply 2.1 and change the tid to the id of post 1 (leave the pid as it is).
  
• If you want be a nitpicker, you should also copy the content of the field 'time' of reply 2.1 to the field 'last_answer' of post 1.
  
• Save

Although the procedure sounds cumbersome, if you react quickly (immediatelly after a new thread is started) it's not that difficult.

PS: There's also an old thread in the German 1.x forum with the same topic.