A Coconut product called Powerstance by Coolstance. My horses eat the coconut meal and that is actually excellent but if you don't want to change (which I highly recommend) you can give her that and she will safely gain weight without any effects on her gut.
Beet pulp , senior feed and corn oil. Feed extra too. She will gain Plenty. This brought a rescue horse of mine back from the brink of 'too skinny'
Rice bran. Step 8 is also a great product!
If your already using seniors feed, try a bit of vegetable oil added to grain, about a cup per feeding
Sorry, not very pulp, beet pulp
Coconut oil is good. You can also buy specially made senior feeds that are designed to put on weight. I also have friends who use very pulp (which must be fed carefully) to put on weight