The limit of sims per household in CAS makes it hard to merge single sims of another household if the number of both HHs exceeds eight.
Let's say you want to copy two sims that are in a 5-sim-household (HH1) into an already existing 4-sim-household (HH2). With a total of 9 there is no way to merge them in one step. You'd have to create a new HH3, loading HH1 into it and deleting the other 3 sims. Only then you can merge HH2 and HH3 to get the desired result.
It would be easier if we could just download specific sims from their HH by selecting them in the HH window: