You will have to use MODELLING.
So ... load the style which you want to install a bass line into.
Go to STYLE - VIEW
Press F3 (BASS) repeatedly until you see BASS BANK.
Turn the data wheel to go through the various bass banks ... you can be playing the style while listening to the various bass lines. If you want to apply this change to the entire style at once, make sure you change SINGLE to GLOBAL (U510 below the screen).
WHen done, save new style.
Hope that helped.
Thanks,