Just tell iTunes not to organize your library and authorize the machine and you should be good. I do this often with playing iTMS video from DVD-R on my iBook.
Unless of course you wanted the SD to be hooked up to an iPod, in which case you're SOL.
Burn the purchased songs to a CD-RW, then delete them from iTunes and re-import them from the CD-RW. Then you can place them on your SD card, and you can reuse the CD-RW again later when you purchase more songs. Thats my method...