That would depend slightly on which version of SMF you're currently using and what exactly you're doing. It's currently only possible to do a full conversion of an SMF board to a phpBB board. All users/posts/forums would be taken over. Merging some SMF information into an already existing phpBB board is hard.
If you're using their currently latest stable 2.0.x line, you can use this convertor
to convert your board
from SMF to phpBB 3.0. From there, it's just a case of simple file replacement to upgrade to phpBB 3.1