Can I put a road bike tyre on a mountain bike?
They make slick tires for mountain bikes used as commuters (and fat slick tires which the air provides a lot of cushioning). Some examples are Schwalbe's fat franks or big apple's (these are more baloonbike) or Marathons . Thinner doesn't necessarily mean more comfortable - in fact, the smaller the tire, the less comfortable it will typically be (this is a function of the bike's geometry and setup as well as the terrain and tires and tire pressures though). Go to your LBS and ask them for tires for road use or commuting and they will probably point you towards something similar to the Schwalbe big apples. Currently on my mountain bike which i use for commuting, I use Geax Evolution tires since I got them very very very cheap (though something like their street runner or some of the Kenda Kwick tires are probably something better for just road use if you're cheap).  

You can't just take a mountain bike wheel and put it on a road bike and vice versa - the brake mounts and what not have to match up (this has been discussed a few times on these forums, whcih you can find with search) which is difficult if they're running different sizes.