I too keep an iTunes library on a Mac in lossless, and need sometimes to do some nips and tucks. Garage Band doesn't have the tools. There are, however, a number of software tools available which will read your iTunes files and give you what presume you want. The one I use is called Sound Studio and is available on the net for a modest cost. Best of Luck.

I'm not entirely sure if it works with Apple Lossless, but Audacity is a great editing program (and it's free).