GarageBand has included Audiobus support along with other enhancements in the latest update. Audiobus is an app that allows you to connect multiple audio apps together at once, experimenting with new sounds and recording your own music.
• Play and record music apps supported by Audiobus directly into GarageBand*
• Turn off grid snapping to get finer control over region and note editing
• Fixes an issue that resulted in feedback while connecting 3rd-party audio accessories into the headphone/microphone jack