It is complicated. It's technically possible to add code to a module using vba, here is an example I favorited a while ago:
http://www.cpearson.com/excel/vbe.aspx
What you might want to do is export the module you want to use as a file, and then import it into your projects when needed with code, rather than using an excel file with the module already in it. That is something that is easier than up above, and possibly more flexible that using a template like you suggested; I've done it with Access databases, and I assume it would work as well for excel.