Custumizing default.Popup, MMB

I’m looking for a way to add a tool, like Pan or Orbit to the MMB popup. I looked an it’s selected to use default.Popup, but how do you change what that does?

I have my middle button set so I click and hold for Orbit or a single click to display the popup, is that what you’re after? you might need to experiment with the Context menu delay to get it to work how you like.