How to set the name for an 'unknown' file extension?

Specifics:

I have various file extensions that are recognized by my mac. I would like to know how to create a list of applications that can open it so I do not get a 'There is no Default Application to open this file' error. I would also like to be able to give these file extensions names recognized by the mac.

Just like .txt is a "Textedit Document" and .html is a "HTML Document," I'd like to be able to change the name and assosication of a file and give it its own .icns if possible.

As for the default applications, I know how to set a default application. However, if a mac does not recognize a file extension, adding a new default application to run the file wil actually erase the old one, which is a big pain, especially when your file can be opened with ANY text editor.

I understand that this may involve hacking into the operating system's databases, but if anyone can help, it would be greatly appreciated.