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?
A file extension does not define the file format, it's just a hint to the operating system (and to the user) of what that file should contain and the OS can have a list of programs that should be able to use this file.
Because the file extension is only a hint the actual content of the file can still be anything, it does not have to match the format that the extension suggests. For example you can create a text file and rename the extension the .bmp, this will hint to the OS that the file is a bitmap image but the actual content is text.
Whatever extension you register, as long as you know how to read the format then you're fine.