Selling Modified Free Extension

Bitpay No longer making extension that works with OpenCart store. I have changed code in free extension to work with OpenCart 2.1.0.1

Can I sell this extension in the Opencart Extension Store?

You'd need to ask the OpenCart team directly. Use the Support > Contact Us link in the menubar above.

My guess would be no, unless the free code was specifically licensed to do that (e.g. MIT license).