Pass a filename to an open application

Is there a way of passing a filename to an already executed application? Like if you wanted to open a text file with notepad you would specify the filename as the command, is there any way to send a new command to a program? I hope you know what im raving on about!

You could try using AppActivate to activate the App, then use sendkeys to send keystrokes to it. With sendkeys, send keystrokes to make it open a file.