Hooking Keystrokes

Question

Hello, I'm trying to create an application for real-time keystroke modification. The idea is to intercept keystrokes in real-time, and then replace their values with other predefined characters. While I can hook keystrokes to determine what the user presses, I don't know how to modify the keystrokes before they reach the application. Is this even possible in C#, and if so, how? P.S. : I realize that there is potential for someone to use this information in hack attempts, but frankly, there are other ways of messing up computers. I assure you, my intentions are benevolent.