I made the couple first and ended up doing the daughter separately (an afterthought), so now I want to bring her into the family. I tried the adoption thing and I discovered, as you said, you can only choose age or gender.
Before I shut down for the evening, I made her active and she went to "visit" her parents. There wasn't time to see if I could do anything because I ran out of time and I'm probably too tired tonight to try but if it doesn't work, it's no problem because it's on a separate save.
I'll try your method tomorrow.