One old, outdated website on Civil War-era model warship combat offers an idea for simulating cannon smoke. This is intended to be used with CO2 powered ball-bearing cannons used in model warship combat, not an electric airsoft cannon. I have never heard of anyone actually trying this, so I have no idea if it even works, but it may give you some ideas.
Guns can be made to simulate cannon fire by using a simple system. When firing, CO2 is released into a plastic pill bottle via 1/8" hose inserted from the top of the bottle 3/4 of the way down. The bottle is filled 1/3 of the way with talcum powder. At the top of the pill bottle, ΒΌ" hoses are connected to the gun ports. The CO2 blows down on the talcum powder creating a cloud that shoots out of the gun barrels. This creates a realistic simulation of actual cannon fire.