So, you’ve got some old GarageBand projects that include electric guitar parts you recorded with a MIDI keyboard and the built-in software instruments. Want to bring them into the modern day and use GarageBand ’09′s new amps and stomp boxes? GarageBand ’09 doesn’t provide an obvious way to do it, but yes, it’s possible.
Select the track you want to convert in GarageBand and choose Edit -> Add To Loop Library. In the sheet that appears, name the track, assign any descriptors to it you like—Guitars, Elect Guitar, and Grooving, for example—and click Create. The track will be added to GarageBand’s collection of loops.
Click the Plus button to add a new track, choose Electric Guitar, and click Create to create a new Electric Guitar track. Choose Control -> Show Loop Browser to expose GarageBand’s loops. In the Search field at the bottom of the browser enter the name of the track you added to the loop library. Drag this track into the new Electric Guitar track. When you do, it will be converted to a real instrument (digital audio) track. Once in the Electric Guitar track you can apply GarageBand’s amps and stomp boxes to it.
If you have a real instrument track to begin with rather than a software instrument, you can simply create a new Electric Guitar track, select the contents of that real instrument track, and Option-drag it to the empty Electric Guitar track to copy those contents to the new track.