There are 2 types of CALs:
1. SMB - Small/MEdium Business
You cannot mix and match licenses. Is your BES Pro at it's licenses limit? How many users are currently on that BES? Do you mean BES Professional or Enterprise? Those are 2 very different versions.
Once a BES has an Enterprise CAL you cannot add a SMB license. The order goes like this:
2. Upgrade - Used when going from SMB/Express to Enterpise
Mixing and matching does not work. If this is not your case then provide as much detail as you can about both BES.