I’d imagine that since lockers are, well, lockers, they should have a tumbler built in for you to set a three-number combination.
If you wanted to take the Rust approach, you could also have separate craftable code locks that you can place on stuff like doors or other buildables, eliminating the uselessness of wooden doors/windows/storage currently.
Since world buildings in 4.0 will have doors, this may play out in a very interesting way.