>>>>later in the day it was sunny---huge number of bees going in and out of the front- almost a cloud<<<<
That was robbing by other hives. If you don't remove the front feeder and stop the robbing, they will be dead in 2 days.
>>>>decided to put a couple of empty frames in the supper to see if they will start building comb on it.<<<<
IF they start building comb, it won't be on the frames. It will be suspended from the lid. You will have a mess. NEVER, NEVER, give bees empty space in a hive. They will fill it with comb if they are drawing comb. They much prefer to draw natural, and will never use frames if they have a choice.