Generally speaking, I believe if the STEP file is okay it should automatically generate a solid body upon import. Unfortunately (and I happen to be dealing with just such a problem myself) if the geometry is inaccurate, you will be missing bits, and might or might not have just surfaces. As far as I know, if you read a STEP file in and don't get solid geometry, there isn't any way to correct things. That's one of the dangers of converting data from one CAD system to another.