as long as it's only for personal use (ie, not for profit), I believe that's ok.
here's how you get the purchased music into FCP:
(MODS - feel free to edit this out, as I don't know if it's against the rules to post this method...though I think I've seen it on the forums before)
burn your music to a cd,
then import it from the cd. You'll wanted it resampled at 48000khz stereo anyway, so this is a good time to import it at that sample rate, in AIFF format. This way, you don't have to rely on FCP to do any extra (unnecessary) heavy lifting to convert the audio on the fly. will sound much better this way, when you watch the finished product.