I tried messing around with something like this about a decade ago. You can't simulate the AoE damage that a chemical/biological agent would have, so you're limited to the kind of kinetic effects of a bomb or missile (direct attack, small area of effect), BUT you can alter the crater and smoke effects. I was able to change the colour of the smoke and make it a little more persistent, but that was about the best I could manage. So technically, it's possible to produce a weapon that has similar visual effects with some tinkering, but without any real tactical value-- anything you hit will still explode as per usual, the only difference being the munition's crater looking like a chemical 'smoke' plume.
...and for the people banging on about the legality of chemical weapons, we still have cluster munitions in this game, and they're currently outlawed in all the civilised parts of the world.