Can I convert a mp3 soundfont to SF2?
Yes and No. An mp3 is a compressed wav recording, usually of an entire performance, sequence, or other audio content. In that sense it's a "finished product", rather like a cake. A soundfont, on the other hand, is a collection of individual sounds, usually used to assemble a musical work. In that sense, it's a collection of "ingredients."
That being said, you could convert an mp3 back to a wav file (an easy way is to use WinAmp's DiskWriter output plugin), then use a wave editor such as CoolEdit to select a portion of the result, then use Vienna or other soundfont editor to include that snippet into a new soundfont. Finally, use NWC to write a new sequence using that soundfont. ;-)