Can you not still befriend them and ask them to join your household? Once they are in your played household, all you’d need to do is adjust the settings for romance/woohoo however you’d like.
I usually create my own line of island protectors living in Mua Pel'am who then regularly commune with the island elementals. I have Keanu and his sister Leilani who are both children of the ocean and islands. Keanu is land protector and Leilani is ocean protector. Keanu lives in a treehouse style wooden stilted house on the Admiral's Wreckage lot and has a son named Tao who he's teaching about conservation and protection of Sulani. I set my island protectors at odds with Aliki and Mele and they are mortal enemies as one side plunders and one side protects, but Keanu is married to their daughter (who I created). So it makes for some family drama. I created them all in the NPC household and then split them off and made them enemies because of their differing values.
I don't know why some ghosts are in Other Households but the island elementals aren't. If they were you could edit them in CAS like I do with other NPCs. Generally that's how I add NPC love interests for my Sims, by editing them in Other Households.
