So Im aware that you can buy (3 hole) spd pedal to (2 hole) mtb pedals so you can own one pair of shoes like this one offered by crank brothers. Has anyone ever seen a cleat adapter that could aid traction when running in a cyclo cross race (i.e. a pair of studs right at the front of the pedal)?
I have never seen a three to two hole adapter that also included toe studs. Even if one existed you still have the problem of inadequate heel tread. When you come into a barrier at full running speed and dismount, unless you have an aggressive heal tread (like on most mountain bike shoes), there is a good chance you will slip and crash. Hitting barriers at speed sucks (I know this too well)!
If cost is an issue I suggest looking for used MTB shoes on a site like craigslist. Used MTB shoes are actually the best as the sole has lost some stiffness letting you run better during dismounts.
Even if you came up with a method of adding a toe spike and heal tread to a road shoe, the typically ultra stiff road style shoe would be very difficult to run in.
Half of the fun of cross is running! Well at least for me :-)