It is possible, but a bit delicate... Processing's command line handling have been patchy for a long time, I don't know if upcoming v.2 fixes that.
Your best bet is to use Eclipse to make a conventional Java program around your applet: with main(), you have full access to the command line parameters. Then you can make an export to get the jars.
In the Windows registry (I assume you use Windows since you mention Word, and associated file type to the extension...) you have to set up the right incantation to launch Java with the right parameters (the jar files and the file name) as default action for this extension. In XP, you can do that also in the File Explorer, I don't know how to do it in 7 (I always did it in the registry...).