Grass hay is good for the turnout. She can nibble on it anytime she likes.
A popular weight-gainer is corn oil added to feed but no more than like 1/4cup. Red Cell is used here for weight-gain, however, I don't like using it. (all it does is increase red blood cells)
Sorry if this isn't much help. Time and regular feeding will help greatly.
P.s. I wouldn't feed her extra ammounds of feed. She should get the normal rate, I believe.
What breed, age, and kind of excersize is she getting? What's her name?!
Can you tell us her story?