Certified GKH Gardening Expert
There is not anything you can really add to make more female flowers. I think the only reasonable piece of advice I have heard that might work is to be diligent about removing male blossoms in the early season (they can be cooked actually and are considered a delicacy). This, in theory, encourages more blooming and more chances for female blooms.
Normally, squash start producing more male than female blossoms early in the season. For winter squash it's still early. Be patient, the male blossoms will fall off but the plants will continue to produce more blossoms. Btw the male blossoms make wonderful stuffed squash blooms, which are yummy so they don't go to waste.