Don't double post, I suggest you delete your second one. These are forums, not chat rooms, it can take awhile for a response.
But anyway...
Basically, just make an opening for a door (make sure the player can fit through the door, I don't know if this is for ESF or another mod) and make walls of another room there. This can be a hallway, or whatever you want.
There's no special technique, it's just like making another room.
*Edit* Found this <a href="http://collective.valve-erc.com/index.php?doc=1046748955-93414200">VERC Collective</a> article for you, it should help.