The driver actually isn't able to inject into already running processes. It doesn't have the code for that. The madCodeHook injection driver only injects into newly created processes. Injecting into already running processes is the job of the user mode library.
Are you doing the user mode injection from within a service? If not, that's what you should do. Here's a demo that shows how:
http://madshi.net/HookProcessTermination.zip