No reason there can’t be another way. Although I don’t think Yahweh will allow the elder gods to initiate the process. Seeing as how Okwari’s time allowed on Earth was coming to an end. Given free will on our turf, if WE initiate the deal, or call the god, it will be allowed to interact & maybe produce offspring. Kind of like when idiots in the books call demons to do their bidding & get eaten.