In my experience withing Europe and in the US, all BlackBerry plans come as two parts. Voice minutes and BIS megabytes.
I have not seen a data only offering from any carrier.
You may be able to negociate a minimal number of voice+SMS text minutes + BIS but I doubt if you will find a carrier who sells on BIS.
BBM uses mininmal bandwidth so there would not be much profit for the carrier in selling you such a small plan and you should never overlook the carrier's desire to make a profit from each and every plan.