You can use call test case and switch window to work on a window previously opened (No way to recognise a manually opened window as previously stated by others)
Have a test case to open browser and go to where you want and do what you want.
Then call a second test case from your first and use Switch to Window, now your second test case will run on the window you previously had open
Not exactly what you are asking for but it was the solution to for me to trying to achieve something similar.
Switch to window doesn’t seem to work. Is there a way to run on existing open browser for debugging purposes? I do not want to run the whole test case again, just from the step which failed. Automated browser is already open till that step, but katalon does not recognize that it is open.