If you're on a corporate BES data connection you might be excluded from adding personal email accounts through the IT Policy they push onto your BB. Nothing for it but to accept it as a limitation, or to get off the BES and remove the IT Policy.
Warning: Do not remove the IT Policy if your device is a company-issued one. And don't try to circumvent any IT Policy. Both these things are usually grounds for immediate termination. Companies have zero sense of humor where their information security is concerned.