Hopefully Simple, Soundfonts Options

[mrmt]

I have a midi composition for piano that I would like to apply a soundfont to, and process out to a wav or aiff file. The standard issue quickime sounds are good but not ultimately what I want. Is there a way for me to add additional sounds, like real instrument samples?

I compose in midi, then painstakingly retranscribe into the keyboard using step programming because there is a problem with my keyboard's midi-in port. I'd rather handle it all inside the machine if possible, without even going to the keyboard.

I realize I'm incredibly novice in regard to midi, so please bare with me if this makes no sense.