extension and plugin in same *.xpi file ?

I have created a plugin for Firefox, which installs with an install.js 
scrip. Now I want to add an Extension to the SAME *.xpi file.

My first tests however revrealed that when I have a install.rdf in the 
*.xpi file (for installing the extension), then install.js is getting 
ignored. If possible, I don't want to make two separate *.xpi files.

Anybody knows a solution ?