Sales Opportunities - Documents

Our customer is using the "Sales Opportunities" funcionality and need to document all documents related with each sales opportunity.

The question is that the documents "Credit NOte" or "Receipt" are not allowed to register.

In the document list only appears below docs. (please see image)

Is it possible to add new "documents" ? Can we add "credit note (sales/purchase)" or "Incoming/outcoming payments" ?

Many Thanks in advance for your help.

Fernando Martins