I assume that you are talking about being added to your corporate BES. Many of us do not recommend putting a personal device on a corporate BES, since the IT Policies can really lock down what you can and cannot do with the device and there will be no personal data on the device at all.
Just adding you to BES would necessitate a BES BB data plan. I am assuming that is what you are asking about. That should not change your number. It might lock you into a 2 year contract however. Just depends on how the company has their contract with ATT.