Yes, I have this done on my 8830WE:
First you might want to read this tutorial:
BlackBerry 101: How to Download and Install Ringtones on your BlackBerry | CrackBerry.com
Once you have created/downloaded the ringtones you want, just copy them to your SD card on your mac...I put mine in a folder called ringtones.
Then go to Address Book, highlight the contact, press options button, scroll down to add custom ring...It will give you a list of ringtones and folders to find the song you want...to set the ringtone for all others, select Profiles>Advanced... and then highlight the one you want (like Normal) and click on options button again and select Show Ring Tones...again a window of folders with ringtones in it will show up, just navigate to the song you want.