I'm having a lot of trouble understanding what your issue is exactly. Are you new Garageband and/or digital audio? Do the song or songs you have recorded open in Garageband? Can you see and hear the song in the timeline? Or are you saying you have these loose files that aren't in a Garageband project and you want them to be?
Garageband saves it's project files as .band meaning all of the different audio tracks(real audio, midi tracks, apple loops) that make up your song are stored in a tidy little package called '.band'. I'm confused when you say "the drum parts are garageband project files and show up as .band files".
Try either double clicking on the .band files or open a new song project and drag all the files you want to hear in that song into the "big track window" and see if it doesn't resemble what you're looking for.
If you can provide a screen shot of the files in question and explain a little more about what you want to achieve I'm certain we can help.