For potions, Estos and Testos are, as far as I know, the Female and Male gender switch potions respectively, so they should work, but I've had awful luck generating that effect.
Equinium+ has a very strong chance to grow a horsecock on a female user, but you can't make that yourself as far as I know.
Using the Cat Dynasty theme room in the Rigard Brothel or the Sacred Spring in the Highlands will both slowly switch your gender towards hermaphrodite, but also change your race to a catmorph or a weaselmorph, respectively.
If you're a female PC, using Roa's "strap on" option at the Rigard Brothel 15 times will cause the enchanted strap-on to merge with you and become a functioning cock.