You can try, but remember that although your lo is tolerating soy through BM she might not tolerate it directly. I would try a very small amount and gradually increase it. There are other alternatives too, like rice, hemp, coconut, almond, milk products.
We don't *need* dairy or soy for calcium, though. There are lots of alternatives (just google non-dairy calcium source and you'll get loads), broccoli jumps to my mind. Same for weight gain if you're concerned, there are alternatives like avocado, coconut oil/milk, fattier but good quality cuts of meat, etc. These are nutritionally sound but give more calorie/fat bang for their buck, kwim?
Hth