
@Chris > Yes you can record using Garageband with the DJM 900.
Personally I would use audacity to record though, since it's a free 3rd party software and has more options than Garageband. Then you can export the file into any format, and import it into Garageband after if you prefer it.
The setup will be similar to using Audacity to record with and there are many threads in these forums on how to record with Audacity.