Alright then.
In the game, the only was to inherit stepchildren is to marry someone with a child or children from a different parent. If you get a genie from a lamp, they're sims with no parents, so Harwood wouldn't be able to make the genie his stepchild because the genie has no parents that he could marry to get a stepchild from.
Adopted children are considered the same as biological children in the game, but there is no way to adopt an adult sim that lives with them already. The most you can do is get a mod that allows you to alter family trees, and set the genie as Harwood's child.
Otherwise, the only possibility is for him to die with a best friend and roommate to live on.