Thanks for letting me know about these page sizes, Rob. We can add them to the list of presets available in Dorico’s Layout Options dialog, but unfortunately this will probably not help in this specific situation.
What Josue might need to do is export the project using the “normal” job type, which should produce pages of 9" x 12", and then use another tool to produce an imposed PDF of the appropriate size. The PDF-BatchBooklet tool available from Philip Rothman’s NYC Music Services page might do the trick.