I guess this might fall under the term "false wall" but I'll do it anyway.
You can use Signs III to make a wall entity.Then you could turn the collision off by changing the main ai script to this:
===================================================================
;Artificial Intelligence Script
;Header
desc = Instant On
:state=0:state=1,coloff
;End of Script
==================================================================
You should now be able to walk through the wall.
Note:I have not tested this yet.I will here in a few.Since the script will work on other dynamic entities it should work on walls too.
Edit Post:
I tested it and it does work.Also you can check in the FPSC media forum to find my thread on making walls that explode.
FLa
Where you can find my demo:http://www.savefile.com/files/6970524