getTab doesn't seem to have that option. addField lets you add a container but doesn't take a type button

Natively, you can add buttons to sublists and forms, but no other elements. Adding an inline html field that renders a button can work, but it’s essentially hacking the DOM, and I would definitely advise against going down that road.

It depends on what you want to do, if you were okay to do elaborate customizations, you could even do a modal window popup that could allow you to further process data.,, Whats the specific reason you require the button to appear on the tab and not the sublist? How do you plan on using that feature, and what's the final use case? How does it help you?

I was really asking for a co-worker, but he wants the button on the shipping subtab because it is supposed to affect data in that tab

It depends on how they're using it, but I'd suggest have them run on the record in the main body as a button, Since unless i'm wrong it's going to affect the entire record.