When creating new weapons, if I want to use icons, meshes and textures already in the game, how do I do that without extracting all of the FO3 data using FO3Archive?

Use FOMM's BSA unpacker instead so you can specify which files to unpack. Once you have pointed to the unpacked files, you can then delete them from your folders.

Just found out a simpler way: just change the name of a base object similar to the one I need to change. Thanks anyways.

Ya, If you'd ask how to make a copy of a weapon, I'd have told you the same thing. If you ever want to mix and match (which is what I thought you were asking), then you'll have to extract the files. Anyway, glad you got what you were after.