4
I'd say it's a good idea to convert (duh, of course I would)
Unfortunately the structure is quite different, but e.g. the users table just has some more fields in e-xoops (IIRC) than in XOOPS.
I manually made a conversion script - but I don't have it anymore - which took the values from one table, stripped the unnecessary ones and then inserted them in the other table.
I don't know if it's possible/recommendable to do with news and forum posts - especially forum posts would be a tricky one.
Can't guide you more than that, I'm sorry.