With this utility, you can load one or multiple loops at once and "tag" them by being able to choose what kind of instrument, music style, descriptor, scale, key, and time signature the loop is. When you are done tagging the files in Loop Utility, the files are saved as .aiff files, but are searchable in GarageBand using the Loop Browser buttons.
The best way to import the files if you have a lot is to do what you would do with Soundtrack loops -- make a separate folder for your "other" loops and place them in the following path Library -> Application Support -> GaragBand -> Apple Loops. That way, they are separate from all of your other loops if you want to uninstall them.
Tip: If you want your own file names for other loops, you have to manually rename them. Loop Utility does not allow for space for renaming your files. Also, there are certain .wav files that will not save very well when they are played in GarageBand. All .aiff files that I have tested work fine. The problem that I have had with .wav files is with any files that are sound effects, but music files have worked fine.

