I have only one plant of each, Zucchini and Butternut squash. Sometimes female flowers bloom when no male flower is open. Can I somehow preserve the male flowers to pollinate by hand later ?
Yes, you can gather the polen in a ziplock bag with a q-tip or toothpick. I am unsure how long polen can stay fertile but after searching for a bit I found a similar question: https://gardening.stackexchange.com/a/23095/20219
After a few months, the pollen may lose over 50% of its vitality, but applied heavily enough, it will be sufficient.
I have had issues with my zucchini growing to about 2-3" and then falling off the vine, because of lack of pollination. I have found that if I go out early enough in the morning with my handy-dandy q-tip, I can find a male flower and take the pollen out and brush it gently on the inside of the female flower, and I now have 3 zucchini over 4 inches in length. I've had better success hand pollinating than waiting for the bees, which I do not see anywhere around. I'm now storing 3 male flowers in a baggie in the fridge for a few days, in hopes of having more female flowers to pollinate. Best of luck to you, as I can totally relate to your issue!