Then, Command-select all of the files you want to convert in your iTunes library and go to Advanced > Create AIFF Version. This will make a duplicate version of the file at the correct settings for GarageBand. If you get confused which copy is which, look for the column labeled Sample Rate. If you don't see a column with that label, Control-click on the Column Headers and choose Sample Rate. The copy at 44.1 KHz is the one you want to take into GarageBand (drag and drop).