How to create own filename extension?
OS X ships with a set of extensions that map to specific applications (and most of these map to Apple provided apps) so you could petition Apple to make the change, but that would be unlikely.

The other mechanism to inject a specific .extension to Application mapping into the Launch Services database is to install an Application that tells the system to map certain file types and extensions to that specific Application.

So, to keep things on-topic here let's just direct you to Stack Overflow to ask how to make your own Application. If you're not looking to develop an application,  you could also just modify the app you want to open your custom file extension by changing it's interal info.plist file:


About Information Property List Files for OS X Applications


Sorry if this level of detail is more technical than you want, but I wanted to answer this for anyone that needs to know how to modify things.