I would add some alfalfa, for protein and weight.
If you want to do pellets, I have been told Dyna Foal pellets are good, and are formulated especially for mares/foals. (I see you are in Arizona like I am, so you can probably get Dyna Foal pellets).
When my mare was pregnant, all my vet really told me to do (since I was feeding alfalfa to all my horses, including the pregnant mare) was to give her a couple pounds of oats a day, which would have the effect of adding phosphorous to her diet (because alfalfa is high in calcium and low in phosphorous, so the oats would help balance it out, since they are high in phosphorous). But I have a friend who purchased a pregnant mare at the same time I did, and she didn't even do that- just fed straight alfalfa and her foal turned out great.
BUT, me being me, I bought my mare a Purina Nature's Essential's mineral block (free choice), a white salt block (free choice) and made sure she got at least two pounds of oats a day. But I was also feeding about 80% alfalfa (and just a touch of bermuda so she would always have something to much on).
So that's what I did anyway. This was my first foal. Very stressful! I worried myself sick over everything. And he had a couple of health problems. My neighbor basically did nothing extra but make sure the mare kept her weight up and she had no issues whatsoever. Go figure!