Sure, you can make it a mash.
Be very careful with going out and buying all this new feed to help your mare gain weight. The more you piss around with her eating routine and what it is that she is eating, you are going to disrupt her digestive system and could make things worse.
I suggest you sit down and have a good long converstation with your veterinarian about what your mare should be eating and not be eating. They know your horse better than any of us on this forum do.
Go with 1 thing, and stick with it. Don't add this, and take away that and then take away what you just added and then add another - find a feed that will give your mare all the best it can give, and then leave it at that.
That's why I like Complete Feeds.
I love Purina Senior: Purina Horse Feeds - EQUINE FAMILY
And I also love what Purina Strategy, Healthy Edge has to offer as well: Purina Horse Feeds - STRATEGY
There are quite a few TB's at the barn I board at and many of them started out ribby and gaunt. Their owners switched from the garbage they were feeding to Senior, and their horses are at beautiful weights - of course they added Pro-Biodics and Pre-Biodics into their daily feed regime, plus upping their hay intake and pasture intake.
I can't stress enough, to just go with 1 feed regime, and stick with it. Don't mess around with it too much, TB' systems are finicky with metabolisim systems that burn off far more than other breeds, due to they are created - mentally and physically.
Start off with fixing the digestive system, then start introducing a new feed slowly, while upping the hay intake.
Talk to your vet :) I know it is hard to get weight on a TB. Been there, done that, deal with it daily. I know you want to go out and buy all this new stuff to get the weight on - I wanted to rush out to the feed store as well to buy anything suggested to me...but it is a waste of money and effort if your horses system wont process it or digest it properly. I can't stress that enough.