I raised my birds from day old chicks. They have had the advantage of enjoying a chicken run since June of last year. When we got our first snow covering the ground, my girls basically just stood there looking confused - and then they all ran back into the coop! I guess what I am saying is that maybe your rescue chickens are not all that abnormal in being confused by setting foot on snow for the first time.

Speaking of chickens out in the snow, last week I had to go inside my coop to break free my pop door which was frozen in the down closed position. As I was going into the coop, one of my chickens literally flew the coop and landed belly down in a 3 foot snow bank. She was so surprised that she was "frozen" in place and would not move. I was concerned that I might have to chase her all over the yard, through knee high snow, until I was able to get her back into the coop. Fortunately for me, she had no desire to move at all and let me walk right up to her, pick her up, and carry her to the coop. Last summer I had a few escapees from the coop and it took me about 20 minutes to chase them down and get them back into the coop.

One thing I learned from my chickens this winter is that any chicken scratch thrown into the chicken run with snow on the ground is basically wasted feed, or compost material at best. My chickens will not scratch the snow and any feed in the snow is basically lost. I do have deep litter in my coop, so I just throw my chicken scratch inside the coop and the girls are happy scratching and pecking inside the coop for food. I have recently thrown a couple bags of fall leaves on the snow in the chicken run and my girls will walk on leaves, and scratch around a bit. But they have no desire at all to set foot on that white snow. I don't know if all chickens are that way, or just my hens. It is funny to see them walk right up to the snow line and stop dead in their tracks, and turn around. It's really like an invisible fence and they will not venture onto the snow.
OK, long story to get to the point that maybe if you cover the snow in your chicken run with something like leaves, straw, wood chips, etc... your rescue hens might feel more comfortable in the run. Depending on where you live, spring can still be a long way off.