You can easily drag a pane or tab out as a separate window in iTerm2. Just left-click the header and drag it outside its original window. However, you cannot simply drag it back.
The secret key combination to drag a pane back to a window or to the tab bar is:
command + shift + option, while dragging the pane body (not its header!). View the screencast below and you’ll figure it out:
You can find the screencast in full resolution here.