You wont have any problem adding Blackberry Messenger to your contract for an extra cost of £5.00 per month. Give Customer Care a call and they will be able to add this for you or recommend an alternative equivalent tariff to include Blackberry.
Will you still be using the device in an iPhone, or will it be only used in a BB?
If being moved into a BB you may be able to ask one of the tech team nicely to take a look at your account and see if there is a way to move sidewards onto a same priced BB plan rather than an iPhone one. This would normally include the Blackberry services, at the cost of less minutes/texts/data.