To shoot steam for the purpose of DI, it's best to shoot it with the same colour of the background of the food. Steam is a little transparent especially around the edges more so for the softer/lighter/thinner ones. So if you shoot it against a black b/g, the black will come through and the DI around the edge will need more time. If the background is not a fix colour or more than 1 dish, than the next best choice will be shooting on white b/g, yes shooting white on white is hard but with proper light control set up it's possible. Maybe can keep the b/g 100% white and the steam 80% white.
If you want to create steam using DI, I'm not too sure but maybe you may want to use the brush tool and than blur it with another layer. From what I know, to create steam with DI for the purpose of Food shot is rather diff. As what ortega said, shooting steam will be easier.