I found a helpful tip about adding a bass line in Audacity. The same trick might work with other recording software, I don't know.
I'm not sure how well it works, I tried it a couple of times and it sounded ok.
But if you record the bass line with your regular guitar, then go to the effects menu and use the pitch scaler to lower the pitch by 50%, it ends up sounding (kind of) like a bass guitar.
You'll never get a true bass sound, but for jazzing up a tune, it might suit your purposes.
The real musicians and recording experts here will likely wince reading that, but if you can't afford to buy a bass guitar... what the heck!
If you scroll down the link below you'll find the information.
http://www.agnula.org/documentation/...ials/audacity/
LADSPA Hint: If you have a guitar but no bass guitar, and you need to lay down a solid bass groove, just record the bass line from your guitar then use Steve Harris's excellent LADSPA pitch scaler to drop the guitar's pitch by 50%. Voila, instant bass ! This effect works particularly well with an acoustic guitar but can be used with an electric just as well.