Can you revise your question as it does not make any sense and I don't know what you are asking for. Or the answer is no, you cannot create a new API on our platform.

Extension has nothing to do with creating an API or consuming a RingCental API.

If you are talking about consuming RingCental API in your application, then extension has nothing to do with it.

Extension only required as a parameter to pass dynamically in creating Oauth tokens.

example:  username=<user_name>&password=<password>&extension=101&grant_type=password

You can pass extension dynamically here : extension=101/102/103.... so on