Keep in mind if the old board returned, it may still remains as a separate database, though. I could get around the 100 MB 1and1 database cap by moving to another host, but to merge two SMF db in the first place it is a major hassle.
You need SMF 1.1. We have 1.1.11
Convert both forums to MyBB.
Use the MyBB merging tools to merge both databases together.
Convert back to SMF
There are no SMF tools to merge two databases together.
But this could wipe out the entire downloads section, some member profile fields, the entire arcade and maybe even forum attachments. It would be a lot of hassle, really. Converting TIMGUL from IPB 1.3.1 to SMF was alerady hard in the first place.