Here's an Applescript droplet that will use iTunes to convert any files (or folders of files) dropped onto it into Apple Lossless format and add them to your iTunes library. If the files have a .flac or .ogg extension, it will call the command line FLAC encoder or oggenc (part of the Vorbis tools) to convert them to WAV first.
I'm using this to convert FLAC concerts from Sharing The Groove into ALAC for listening on an iPod.
(I'm not the greatest Applescripter, so it's not perfect, and there's no error checking. Drag it into Script Editor to edit the code, and if you make any useful changes send them along and I'll incorporate them).