The quickest/easiest thing that I can think of (If you can not find another way of mapping forums to users), is to create a temporary_id (temp_id) column in your users table.
The temp_id will have the same values as the id (in the users table) and it will just be a INT column.
One you got that, you can then import stuff into your new DB. Then, write a script that will use temp_id to get the right forums for each user and update its id with the new users id.
Hope that helps