The other ones already override the base game ones.
There is no fiddling with scripts if you install the scriptorium by Numenor. Install Scriptorium, then drop any text file in the folder it creates in your download folder. Its easier than installing one of the EPs.

And it isn't any different than downloading cc and dropping in your downloads folder. The only way to get new stairs beyond what Maxis has is to use script files, since the Maxis ones themselves use script files.
Anything can be cloned and made to override anything in game. I don't see the value in losing those glass stairs though, sorry.