Unless you want to completely rewrite Combat Arms from the ground up, a singleplayer version of the game would either be absolutely awful, or completely impossible.
The main purpose of such a feature would be to gain exp and gp. However, this could be so easily exploited that loads of gp and exp will flood the system. Remember, if something is offline, it's much easier to abuse it with hacks and the like.
If there were to be a "mission" mode that didn't offer any extra gp or exp, I doubt anyone would play it more than once, just not practical.
Beyond that, the engine frankly isn't built for SP. A better option would be the ability to play multiplayer matches with artificial intelligence, ala Quake 3 or Counter-Strike as opposed to a full Single-Player mode with a story line.