So long as they aren't thin - check if you can feel their spine, you should have to press to feel bones - then plenty of good grass / hay will keep them growing, yes.
Personally, being where we are, we feed a little bit of cake - just 1/2lb / head / day - to all overwintering lambs and find that sets them up better for a long, healthy and productive life in the Cumbrian uplands. I'd do it differently if I lived somewhere milder and with more nutritious forage.