「Andy's Mac OSX Music Page」で、「GRBand Assistant V2.00」というフリーウェアーがダウンロードできます。

GB DrumLoops
GB DrumLoops is a package which is capable of creating drum loops in the Apple Loops format ready for import to GarageBand or any other Apple Loops compatible application.

GB Import
GB Import is a utility which imports standard exs files (as used by Apple's eMagic Logic package) into GarageBand as new Software Instruments.

MidiMaster
Midi Master is a utility which uses information from a Midi file to play music through a sample based synthesizer, producing high quality sound through to your system's loudspeaker.
It also has the capability to render the sound to hard disk in the form of an .aiff file which can be subsequently imported to GarageBand.

とのことです。（訳する気がおきないので、このままで失礼 ）