Move threads to other site/forum?
 

Is there anything available that would allow moving threads from one forum/site to another?

Or would it be possible for example to merge user accounts first, then move certain threads?

Impex...

Impex imports everything right, and overwrites? I want a more selective procedure.

I don't think it overwrites, it just imports and adds the content in incrementally I think.

ImpEx will import it all - you can't just say Only Users From Registered Usergroup, or Only Threads from ABCD Forum. You would need to import it all and then delete what you don't want.