Can you prepare orzo coffee in a moka pot? I have a Bialetti Moka Express and would like to try orzo/barley coffee.

I know they sell an orzo express model, but was wondering if I can just use my moka pot
Yes.  


With coffee moka, you have to fill the funnel with barley only for one half;   
Otherwise, you can use the barley model, named as Orzo Express, they have the funnel shorter and you can fill entirely without problems.  


Using a moka for coffee, and a different one for barley coffee, is a good practice, otherwise the taste of (ground) coffee will be highly altered.