If by Mimosa you mean this tree
It's Latin name is Albizia julibrissin, also known as the Silk tree, (although it does not produce silk).
I've read a couple blogs that suggest the "peas" green stage only, MAY be edible. I would not feed more than a few and see if there are any negative side effects. This is just smart to do on any new food source you're not sure of. Wait several hours, even overnight, if no problems try a few more.
As these blogs emphasized the green shelled peas from the pods, not brown (said were hard as a rock when they're brown), I don't think you could save them, (unless maybe you could freeze them). But do a google search yourself and see what you can find. If this is the same tree you're talking about, just make sure you use the Latin name in your search or you may get an acacia tree reference instead.
Another source suggested the young leaves may help color up the yolk of hens eggs. I don't think it said what color though. And also certain other livestock like goats could eat the young leaves too.
Whether it is edible or not versus poisonous seems not widely studied, at least from the quick search I did, maybe you'll have better luck if you spend some time "digging" good luck.
They are a beautiful tree, I am enjoying the hummingbirds getting nectar from the flowers on our tree.