One of Safari's useful functions is being able to manage tabs either by organizing them in the same window, or by dragging them off into their own windows. Though this allows for some organization, one missing feature is being able to consolidate specific windows into the tabs of one window. Safari has options to do this for all windows, but not for individual windows.
To consolidate all windows into the tabs of the foremost window, just select "Merge all windows" from Safari's "Window" menu. If, however, you want to move a specific window into the tabs of another one, it can be done, but it takes a couple of more steps.
The key to doing this is to treat all windows as tabs, which can be done either by showing the tab bar at all times (press Shift-Command-T), or by creating a new tab in a window that has none (press Command-T).
With the window contents available as tabs, you can drag the desired tab from one browser window to another one, even if the destination window does not have tabs enabled. When you drop it, Safari will insert it as a tab in the target window.