you can do that, but you will get tired of switching wheels, and you will need a whole new rear wheel with cassette and hub etc.
This is Greek to me ... keep in mind that I am a complete novice ... feel free to talk down to me ... could you not just have a second set of wheels with true mountain bike tires that you put on when doing single-track off-roading? Is it really that hard taking the wheels off and sticking on different wheels?
Bill