What to mix with formula besides water to make taste better?

My daughter is about 7months old, and my wife's milk supply is
starting to diminish.
We started mixing cereal with her milk to bulk it up about a month or
so ago, and she took to that readily.
We have been trying to find something to help supplement her
increasing diet, especially when she is hungry at night.
Although we have had some success mixing small amounts of formula with
breast milk and cereal, she will not take the formula when mixed with
water.
We have been trying the Enfamil lacto free lipil (which my son took
with no problem when he was an infant, but I guess my daughter is
pickier)
Clearly we want to find something she will eat besides breast milk so
that we are prepared.
1) Is there anything else we can try to mix with formula besides water
to make it better tasting? I am wondering about soy milk, or rice
milk.
2) Anyone know of a better tasting formula to try?