You can export GarageBand for iOS projects from your iPad and copy them into your computer to edit using GarageBand for OS X. The reverse is not the case. Full GarageBand projects can be too complex for the iPad and may use resources not included in the iOS version.
But, a method that may work enough is to share the file out as a song from the Mac, then add the file to iTunes, and sync it into the iPad. Then you can import that song into a GarageBand project through the "Loops" menu.
It worked enough for me.
All the best