While looking through the xml's for plants and opening up all vanilla plants to be farmed, and hyperweave cuz why not lol, i had a thought. Rather than the story about the stork, country people around here say that when a women goes to a cabbage patch and takes a head of cabbage home, that head of cabbage will turn into a baby. So, why not create a crop similar to lettuce / cabbage, etc, with a high fail chance, low hit points, long growth time (9 / 12 months in real life so 3 / 4 seasons in rimworld?), etc, when harvested at 100% has a chance to trigger the event where a random colonist joins? While i'll probably consider it at some point, hence it being a concept now, anyone's free to use this idea as i suspect it would require C# to make possible and i have no C# experience at the time of this post.
Someone did suggest having "vat babies". This seems similar along those lines.