Can you assign ringtones to BB IM?

I would like to assign a ringtone to one user in my BB IM so I am notified when they send me a message. Can this be done and if so how? I would also like to assign a different tone for another user and then a generic tone for the rest of my department. All of us have BB's but different models etc. I am sure others will want this option if I learn how to do it.