Adding new field to Content REST API datacontract

Is it possible add an extra field to the document json datacontract served by the Content REST API? Is there a way to extend the Content REST API?

I know this is possible with the HAL Content API, but I want to be able to add an extra field to the json documents from the Content REST API.

Content API is very limited (on purpose)…
you would probably need to add or override some of the original configuration: