Re: There should be a way to merge builds instead of just replacing them
Yes, absolutely what @puzzlezaddict suggests. This does work, I use this method a lot to create retail lots and retail chains. For example I've merged several different small fully stocked retail lots into one to make a shopping mall by saving each room from other stores and placing them on a new lot.
I've also taken rooms from other retail lots to make a totally new store with ready made stock.
I've used it to add rooms to an apartment building/house share as well and then copy them throughout the building just by using the copy room function.
So it works for any type of lot whether business or residential and it's pretty straightforward to do.