I do this all of the time. It is super easy in Garageband to play and hear the song with bass and drums in GB. When you hear an area of the song you may want to modify or loop etc… Use the SPLIT and JOIN commands to slice up parts. I.E. Lets say I just want a 4 bar drum part of a midi song I imported from the internet. Just import the song into GB. Find the drum track. Highlight it. Use the timeline marker and place it on the 1 bar marker. Use the SPLIT command to split the long drum kit. Highlight just the small 4 bar clip and follow steps above to add it to loop library. It even has an easy tool to LOOP in Garage band - called LOOP REGION- to hear what the split/edited parts will sound like in BeatBuddy. You follow the same steps above for a split clip. Just highlight the split part of the song you want and follow steps above.