A new-field hotkey doesn’t currently exist, though I agree that it would be nice, and would be good to request in the Product Suggestions category.
As for making one using a macro tool, that would be difficult. Making a new field currently requires a click on the new-field “+” button, but the placement of that button changes depending on how many fields are currently active/visible in the table. If you’re not in a Grid view, it’s not even visible. Assuming that most macro tools record the screen coordinates of mouse clicks, it’s not guaranteed to work because the button position always changes.
