Hi,
I need additional functionality which allows us to place one topic in two ore more forums.

In my opinion it's a big change that affects forum presentation, search engine, and topic management.
I'm planning to add an external linking table (topic_id, forum_id).
I'm affraid the only way is to modify your sources without possiblity of future upgrades.

Can you help me to analyse this solution? I would be thankful for any tips.

That's a pretty big idea... sounds possible.

I think this would be nice particularly for Rule topics. I hate having to repeadetely post the same thing in multiple forums that require similar/same special rules.