Help for replacing a .js file in comm.jar

Hi,
I'm working on a patch and need to know if I can use xpinstall
to replace the file navigator.js that is inside the comm.jar file.

I have read the xpinstall reference and some kind of tutorial, but
could not find a funtion to replace a file that is inside a .jar
And the other cuestion is, if this is posible, can I do it while
Mozilla is running?

If this is posible, is there a howto or tutorial available?

Many thanks
Oliver