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I'm in the process of doing just this at the moment. I've written a Python script to migrate the users and the forum posts from phpBB2 to Xoops/NewBB and it all works pretty well.
The biggest problem I had/am having is that the phpBB2 BBCode is richer than Xoops. The conversion can take care of most things, but I have had to hack XOOPS a little to get support for [list] type constructs in posts.
When I am done, I would be happy to let you have the conversion script, it is very messy, but works for my needs.
Cheers
Paul Gregory