You could mix the breastmilk with formula, technically there is nothing wrong with that. You might end up wasting breastmilk if you do, especially if your toddler cannot finish everything. You can't keep formula milk that's already prepared and if you mix breast milk, you might end up throwing away precious breastmilk.
Also, do not just directly mix the formula powder with the breast milk. This will change the composition of the breast milk, the entire mixture might become so much more concentrated that it can be bad for your toddler's kidneys. Always follow the exact instructions of mixing the formula first, before adding the breast milk :)
More info here:
http://www.babycenter.com/404_can-i-mix-breast-milk-and-formula_8883.bc
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