has an application called SCMLoader that can load a mod on a per-map basis. In other words, each map can have its own mod, but the player must run the application for it to work. http://sfsrealm.hopto.org/downloads/SCMLoader.html
FireGraft's supply edits work fine. Just know the values are actually doubled, so max is 400 in the editor but will show as 200 ingame. This is because Zerglings actually cost 1 out of 400 rather than 0.5 out of 200.
Since I know nothing of your project, like whether you intend to use modding or not, my suggestions are limited. If you want a pure UMS solution, make the supply limitation a part of the map's gameplay. Maybe too many supply depots will attract mutated critters that will eat up a supply depot, so you either have to keep below the danger level or station forces at home to kill off the moochers. Similarly for Protoss or Zerg, some psi-gobbling critters would be attracted. Have them come in waves, and the more you are over capacity, the bigger the critter swarm will be to offset the army size gains.
Alternatively, balance your resource acquisition. You can't build a large army without funds. Give ways of retrieving and repairing existing units, rather than resources for replacements.
Another way is through map design. Unless the race is Zerg, supply eats up a lot of real estate. If players have limited places to build, they are by default restricted on supply. If the race is Terran, you could make use of lift-off. Certain bonus spots/pits might be dangerous, damaging buildings in it every once in a while (flood, lava flow, earthquake, some subterranean critter), but lifting off will prevent it.
Or like Church said, make units cost more supply. The map might be so hostile that equipment erodes and breaks frequently, food rots, and other nastiness that simply requires more supplies to keep your units operational.
: The map editor allows you to change a unit's supply cost already, doesn't it? No need for DAT editing.