It honestly depends on not only the company, but also the region. For instance, Comcast does not have a policy company-wide that allows customer installed CableCards, but the region I live in (Sacramento) does. So I'm able to pick one up from a retail location, go home, call them up and pair it over the phone.
You should just call the local number and see if they allow it.
You should just call the local number and see if they allow it.