Can mashed bananas be bagged and then frozen?
It depends on the recipe.  Freezing and thawing bananas will result in a softer banana purée, significant separation of liquid, and browning. The longer they are in the freezer, the more pronounced each of these effects. For some uses (smoothies, banana bread), they should be fine for up to a few months, though the color will be affected if you care about that. You may want to just break them up into chunks rather than puréeing them before freezing, as chunks are much easier to separate than a big block of frozen purée, and much easier to portion out than full (peeled, of,course) bananas.