Stianless Steel to "regular" Steel

Help, I am trying to attach a couple of fittings to a S/S keg for heating elements. The problem is I can only find "regular" steel fittings @ OSH, they don't have Stainless Steel. Can ferrous fittings be welded to S/S? I have a MIG setup, and when testing on a failed project, (the first attempt on a keg) every thing seems fine, but when I try to weld the fitting to the side, no matter how many times I grind away the weld to find the leaks, I find a fine, "line" between the keg, and the fitting, right in the middle of the weld that goes right though to the base of the weld. Almost like the MIG wire mixed with the two metals, but not with each other.