Thank you for your question,
If you take a look at the following Symphony Developer Forum post - the same question was asked before & answered by my colleague Yong Sheng, which provided a detailed response.
In summary, you can create and develop a new extension application to be used in the Develop2 sandbox environment, however the extension application bundle file will need to be 'side-loaded' locally using the method described in the Forum post above.
This will allow you to run and test your own extension application without having to upload the extension application to the Develop2 sanbox environment's marketplace. Running the extension application locally (via 'localhost:4000') is currently the only way that is allowed for developers building on Symphony.
Please take a look and let us know if you have any questions or feedback.