I'd lean towards more alfalfa and/or flaxseed as well.
Alfalfa, as deserthorsewoman mentioned, is low NSC and high in calories.
Flaxseed is 40% fat, most of which is omega-3 (the good omega
) Since your horse has limited fresh grass (where horses get most of their omega-3 naturally), I'd keep towards things like this which are high in omega-3.
Rice bran, which is 20% fat, is good for putting on weight, but is a little high in NSC at ~21% and since your horse seems especially sensitive to sugar/starch I'd avoid it. (On a side note, has she been tested for any disorders other than PSSM?) It's also not great in terms of omega-3 to omega-6 ratio.
Oil can be a good choice, but ideally you'd feed flaxseed oil for the omega-3's and that can be very pricey. Canola oil is the cheapest, easiest to find oil with OK omega balance (still higher in o-6 than o-3, though) if you want to give oil a try. Some horses object to the texture, in which case mixing it into a mash might help- if you currently soak the alfalfa pellets you could add it to that.