You can share an Event with Persons/Families (other than those with the pre-defined Primary or Family Role). Use the Share button (in the Events tab of the Winess’ Edit Person dialog) to select the Event for the object selector dialog. Or drag the Event from the Primary Person to that Witness’ Event tab.
An Event Reference Editor dialog will appear (for the Witness) with the Role set to Unknown. (Instead of the Primary default role when creating a new Event. As shown in the screen capture below.)
Change the Unknown role in the Event’s Reference Editor. For a Witness, you do not even need to add a Custom Type.
Pre-defined Types of Event Role: Aide, Bride, Celebrant, Clergy, Family, Godparent, Groom, Informant, Primary, Witness, Unknown, custom.