Some unzip tools enable to extract xpi files.
I'd be glad to know if there's a program which also enables to compress files directly to xpi.
An XPI (XPInstall) file is simply a ZIP file containing the extension files, with the install.rdf file at the root of the ZIP.
To do it directly, you will need to add a Registry entry for custom context menu item (say Compress to XPI). It will invoke a predefined .bat file which takes selected files as input and generates xpi for that.
After doing this all you need to do is, just select all files which you want to pack in xpi and from right click menu select Compress to XPI and you will have your xpi file.