A new tab can be added to a Finder window that already has tabs by dragging a folder’s proxy icon to the plus sign that appears in the tabbed window.

You probably know you can drag the proxy icon of a Mavericks Finder window to another Finder window’s tab to move or copy it to the tab’s folder. You can add the folder as a tab to a Finder window that already has tabs if you drag its proxy icon to the plus (+) icon next to the right-most tab of the tabbed window.

Source: Mac OSX Hints