I think this is going to be difficult. Probably the best forum add-on for c5 ishttps://www.concrete5.org/community/forums/customizing_c5/interest-i...

There will be no way to automatically import data from different forum software into it. However, it's open source so the database schema is available to see, and it's actively being developed so you might be able to get some tips. However, I think a fair bit of manual database manipulation will be required.

You could try out the c5 forum and see if you think it's suitable before investing a lot of work in the transfer.