I've seen loads of stuff recently for using your microwave to steam food. I understand the concept, but surely you are still microwaving the food as well?
What do you mean by steaming in the microwave exactly? If, I presume, you mean having food in a container above another container with water, then I reckon the microwaves, which are tuned to the wavelength of water molecules, will agitate and thus heat the water molecules better or more efficiently than the water in the food above. The answer to your question then depends on defining the water content of the food to be "steamed".