Well for one, you need to abandon the notion of visitors from another planet coming here. Instead, these being would be spreading out across the Universe, colonizing. If we are being visited they are not going back and forth between a home world but would be far closer to us. In fact, our planet may exist within their territory.No, I'm not suggesting anything of the sort. I'm just saying it all sounds very terrestrial, in particular as you have described it above. It sounds more like animal husbandry than anything else and seems more indicative of locals tending their herd than travelers from distant worlds exploring the universe.
If you want to breed humans, create being from the sperm and eggs from humans, then you will need to get your hands dirty and get the sperm and eggs.