Does anyone make an overdrive that is compatable with the allison transmission?

I haven't searched for that feature, but here's something else to ponder...


Supposedly the Allison has a built-in slot for a 6th gear. Some of the more pricey RVs have 6 gear Allisons, I'm not sure of they're Allison 1000s or 2000s. The aftermarket folks haven't jumped on it yet, so it must be more difficult than orginally thought.