Importing

Ok. I'm working on a Doom II PWAD, but Im wanting to use the Doom I skyboxes (and possibly other textures) in some areas. So, how do I pull these into my project? I'm using Doom Builder.

DoomBuilder is a level/map editor, not a resource editor, so you cannot do what you want using DB.

You need to get your hands on a resource editor like XWE (http://www.doomworld.com/xwe/) or WinTex(http://www.3ddownloads.com/showfile.php3?file_id=7883), and extract the sky patches you want.

The sky patches aren't skyboxes, and in use they are properly called "skies". DOOM doesn't normally even use skyboxes.

Ok, I have WinTEX, so should I just make a n addition PWAD with the textures I want?

Select a texture sky.
Go to menu Edit > Load entry from file.
Find you new sky texture and...
Done.