copy over files with xpi???

I have an xpi right now that adds some functionality to mozilla.  The
problem is that to do this I also had to edit nsHelperAppDlg.js in the
compnonents directory and nsContentAreaClick.js in the comm.jar file.  I
was wondering if there is a way to get my xpi to copy those files over
the old ones?  I think multizilla does something like this but I can't
see how.  I would like to just package the nsContentAreaClick.js but if
I have to give it the whole comm.jar file that is ok.  Any help on this
would be appreciated.  I would like to distribute this from a website so
just copying the files by hand like I do on my computer is not possible.
Thanks.

Jason