John. Are you just talking just about the 'Zoom' Extents button itself or a custom toolbar. I've never had extents in the right click menu, so I can only assume you added it at some point in the past.
I'll assume just the extents button (the same as if one presses the mouse wheel). Tools menu - customise. Toolbar tab (not the pop-up toolbar tab). tick Pop up_1, or 2 or 3, whichever you want to add the extents button to. This will open the pop up as a separate toolbar. go to the Commands tab. find the extents icon, (possibly under View - Zoom). Drag the button onto the end of the pop up toolbar. Go back to Toolbar tab. Turn off the pop-up.
The Extents should now appear in the right click menu buttons.
Sorry if I have completely misunderstood your requirement.