[2022] How do you expand an existing room in beta?

There are two ways;
Open the designations window from the top left of the HUD or with the [ key (this shows you all your jobs/rooms). You can select a previous room in that list and start adding more room points to it.

Or

With the rooms toolbar menu open, hold left-ctrl and right-click a previously designated room. It will select the room and let you add points to it.