Is there a road shoe cleat adapter (from road to mountain) that could be used in a cyclo cross race?
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 :-)