I'm sorry but you can't. Both forums' topics and posts have unique IDs, if you put those in the same table, they'll be duplicated and data will not be inserted. This is the base logic of all database engines, all rows have unique IDs.

The only way is do this, is the wpForo to wpForo migration. There is no Export/Import tools, it can only be done by migration processes. This is a codding service, this is not an easy task. I've already mentioned gVectors Team services here. Also you can check these services:

http://profprojects.com/

http://gconverters.com/