Here's a couple of methods that I have used, though again, it doesn't replace using a real bass....
Try tuning your guitar to a low 'A' and run it through a ptich shifter. I used the pitch shifter on a SE-50 and wasn't too displeased with the results. Another problem arises with this method however. It is often hard to get clear, well defined notes when a guitar is tuned this low.
My second method; I set my tracker record speed to it's fastest setting, then recorded my bass (guitar) track. Once I was satisfied with with the track, I slowed the record speed down to it's original (or slower) position. This gives a real smooth, and kinda interesting bass sound. Unfortunately this method is not without flaws. It is hard to calculate timing and pitch when recording at different speeds. The easiest solution is to record your bass track first, then tune your instruments to the lowered track.
Dan S. Leonard.