You sure can! I just create a playlist of whatever tracks I want in iTunes, then sync it via the desktop manager. It brings up a screen similar to when you sync an iPod (i.e. you can choose which playlists to add). An alternative and slower way would be to do it via bluetooth, but only if you have nothing else happening in your life...
You can try Jam11 which will enable you to stream your iTunes music off your Mac or PC onto Blackberry. Right now it is free and available on Blackberry App World. You will never need to sync again as your music can be streamed anytime so long as your computer is connected to the Internet.
