They will have to have an available license for you to use in order to connect your BB to their BES.
One BIG thing to find out before doing this is what they restrict or enforce via the IT Policy that is set on the BES. Does the policy force you to have a password on the device? If so, what is the password criteria (length, timeout, # of password attemps before all data is wiped)? Is that something you can live with?
Does the IT Policy prohibit you from sending 'as' your personal account? They can lock that down, too (you can receive from personal, but not send as the account - all outgoing mail can be forced go to from the work account).
Find out if they will provide you ANY support if something doesn't work right. Not getting mail - they may say "Sorry, this is not a work device, you're on your own".
Just a few things to consider.... The IT Poilcy is the first thing to find out, though.
No longer a BES Admin, but it was fun while it lasted!