Is it possible to add an additional board on Arduino Web Editor (create.arduino.cc)?
No, there isn't.

The web editor is completely closed. It's a service run by Arduino, and as such it will only support Arduino's own boards. 

Two reasons:


Why should they spend the time, effort and money adding support for other people's products?
It adds value to their products to encourage you to use real Ardiuno hardware.


Business is business, after all.