GarageBand is an awesome tool for bedroom rockstar wannabees. It provides a slick, easy to use interface that lets you record audio from mics, guitars and other live instruments, and also record MIDI from keyboards and other such devices.
I typically record a drum track on the keyboard (when I'm not using the supplied samples). But playing an entire kit on the keyboard (and keeping the fills in time!) is pretty tricky, so I usually record them in layers. First, bass, snare and high-hats, then toms, then cymbals. And often yet another one for percusson (shaker, triangle, and so on). That ends up being a lot of tracks, and it would be much easier to work with (and extend across many bars) if you could merge them once you've laid down a good pattern. But copying and pasting MIDI data onto a track would replace the existing data, and there is no "Paste and Merge" function. I figured it wasn't possible, but with a bit of persistence I figured out a way to merge MIDI tracks.
First, record your pieces on separate tracks as you need. Once you're happy with the final result, solo the tracks you want to merge and play them so you can verify it sounds right together. Then select the source track and go into the track editor (the "eye" button on the lower panel) so you are now viewing the notes. Make sure that the lower panel has focus (this is important, as it behaves differently when the upper track view has focus) and press Cmd-A to select all the notes, then Cmd-C to copy them.
Then select the target track, click on the track editor to give it focus, position the time bar at the start of the recording, and press Cmd-V to paste. This will add the notes to the existing track. Solo the combined track to verify it, then you can delete the source track. Repeat as required for all your other tracks.
So it turns out it is pretty easy to do, once you realise that copy/paste is destructive (ie. will replace) in the upper panel, and is non-destructive (ie. will merge) in the lower panel. Just make sure it really does have focus (the background goes a different shade of blue) and the timeline is positioned at the start. Rock on! :)