The boards table, after doing these things, will have a new column named "ID_FORUM".
The value of this column will determine which forum it is shown on. For example, if its value is 0, the board will be shown on all forums. However, if it is 1 it will only be shown on forum #1. Since no administration interface has been added, you will have to manually edit this column's value using phpMyAdmin.
The "messages" (where the posts are stored) and "topics" (where topics are

) tables *DO NOT* need the ID_FORUM. Only "boards" does. This means you only need to put the posts both into SMF, and it will sort them out for you.
But, yes, the combining part does seem like... a problem.
-[Unknown]