Extends invoice functionality - by: mgribaudo

Hi,
I'm very new in Dollibar.
My need is to extend customers invoice funtionality in order to:
- add a new invoice type
- use a separate counter for the new type
- add some extra fiels to the header

Now, which way can I move?
- create a new module?
- extends invoice module? It's possible?

Thank's in advance

Marcello