Yes there is an adapter, and the good news is it's both a job you can do yourself and won't cost a lot (just over £30 plus postage).
Check out
http://www.expansys.com/p.aspx?i=114502. You have to pull out your existing holder, unplug it from your cark-91 control box, then plug in this new unit and fix it into your car.
I got fed up with legacy solutions, so have now got a Nokia CK-7W in my car which has bluetooth for any supported phone. The best thing about this is although I do have a cradle for charging, if I'm just zipping about I can leave my phone in my pocket without thinking about connecting it. Far less chance of being tempted to answer those quick calls that way that you see so many other drivers do.
It should be mandatory for all new cars sold in the UK to have a bluetooth kit installed, but who's going to make that first step?