How can I record streaming audio on my mac for playing in iTunes?

I strongly suspect that is violating somebody's terms of use and since Apple has to play nicely with the media companies in order to sell media on the iTunes Store we would probably get in trouble for helping you with this on an official Apple forum. Check the TOU link at the bottom of this page and there's something about not posting assistance for illegal activities (i.e., breaking digital rights management -- if Sirius wanted you to do this they would provide download links to the music files).