If you're specifically asking how to add a room beyond the exit door, that's not possible without using BEEMod or Hammer. If you want to add a new room in the base puzzlemaker, the best you can do is to connect two rooms with a one-way corridor.

Yes, connecting two rooms with a one-way corridor, then as a "door" you have multiple options, angled panels, fizzlers, laser fields, grating, or something more creative.

This map for example is using a funnel for navigating different rooms:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3412310030

Extra tip: The lightbridge can also be used to block the entrance to an area if placed properly, you can't jump over it, nor duck underneath. Could serve as a door.

For example the "start line" in this level:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3414087350