As long as you start the debugger beforehand....

Run the program with the debugger enabled, pass c as the command to the debugger. The program now runs normaly.

When you reach the point in your program where you want the debugger activated, press ^C in your console window. Instead of aborting (like it would if the debugger were not activated), the program will drop you to the debugger prompt.

I don't think there's a way to do this for processes where the debugger was NOT activated first.