Knot made a good suggestion with the chainring, although I think in your instance it would HURT shifting. If I have read your description of your bike accurately, you have a 50-34 compact crank.... which means that your front derailleur is at its limit with that setup. You can indeed put a mountain bike cassette on a road bike, but currently with some issues.(if you have a 10 speed drivetrain) Shimano does not make a 10 speed rear derailleur(mtn bike), so although someone can probably make something work, I don't believe without problems. People have mixed and matched Shimano and Sram together, but it does add a bit more noise to your drivetrain, and again, not without some issues. The thing that I would suggest(although costly) would be to add a triple crank. You would have to buy a front shifter, crank, bottom bracket, and front derailleur, but that if installed correctly would be your best bet. Down the road, I'm sure that Shimano will come out with a 10 speed mountain bike group, and then you'll be able to lower gearing even more and have something that actually works.

The compact cranks are indeed an issue for those of us that live in the mountains... they simply aren't low enough.(especially with 10 speed drivetrains) I'm hoping that Shimano and SRAM will realize this, but only with complaints from customers and many bike shops!