Well, I had never heard of XMB until now.
It looks like they attempted a spinoff of phpBB2.
So IMO, you would be able to run phpBB2 and it probably has the mods you have currently installed on your board.
Though phpBB3 has a lot of features built in that phpBB2 is lacking… though it's still in Beta and there is no support for running phpBB3 Beta live, so it's not a good idea at the moment… so you'll probably want to wait until phpBB3 makes it to RC1 before you convert to phpBB3.
There are currently 2 converters from XMB to phpBB2, so I would say the best route to take at the moment is to convert to phpBB2, then from phpBB2 to phpBB3… at that time you will be a happy camper
Here is a link to download the converters
http://sourceforge.net/project/showfile ... e_id=68166
Give both of them a try and see which one works for you.
Just make sure to follow all instructions… you will have to install phpBB2 before running those converters.