I was wondering, if I have a custom extension(One not editable usually, and not set up with registry) like .thescriptergeek, could I still edit the file as if it was a .txt?
Yes. Files are conventionally just piles of bytes, so you can open them in whatever mode you like.
Granted, it's most portable to treat them consistently as either binary or text, since some systems treat binary and text files slightly differently (e.g., in the handling of newlines.)
-- I think you can delete your duplicate thread at this point (although I just replied to it, I deleted my response. Hopefully that means it can be removed.)