Last winter I had one skinny hen - even though they had ad lib mixed grains, OSR, peas, etc, and the others looked great, she was the bottom of the peckng order. They had been wormed too. So every night I brought her into the hall for an extra feed. She was stressed the first couple of times for half a minute, but then saw there was food on the go and started pecking away. She'd eat a good handful and then settle down in the cats' bed to sleep, I'd pick her up and pop her back in the hutch. After a short time she was quite happy to be picked up. One really cold days I just kept her in the hall with the heater on.
I found growers pellets, softened with some warm water into a mash with a bit of vegetable oil worked wonders. She could eat quite a lot of this food.
I did wonder about culling her at the time, but am so glad I didn't, she lays beautiful dark eggs and at the moment is a super broody, and its handy that she's tame.