Unfortunately there is no like mnu for ribbons. But it can be done a cuix is actually a zip file so copy a cuix and rename to .zip you can open it up and it has lots of menu's inside.
The file Ribbonroot.cui is the one your looking for. The xml is very complex and not simple like a mnu.
I looked at this at one stage and what you need to do is write a xml file which is a text file. As far as I know there is no lisp -> xml for this.
Make a custom toolbar via CUI and have a look at what you get in the cuix just make a couple of items so see pattern.
My custom menu's dont have ribbons so its empty but acad etc are full.