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.