Is it possible to add a factory Sirius Radio to Ridgeline Trims that don't have it? I know I seen some for the new Pilots and also where is the antenna installed on the Ridgeline that has the factory setup?
Yes on the audio source on the touch screen there's an option for SiriusSirius sells kits for G1 Ridgelines but it requires an XM button on the radio. I don't know if the lower G2 trims have a SiriusXM option on the audio screen or not. If so you may be able to add it, but like all other G2 aftermarket add-ones you may have to wait a while.
Then it's probably worth a call to Sirius to ask if they plan on coming out with an add-on kit like the one at this link for earlier Ridgelines. My guess is it's just a matter of time. Good luck!
Complete Sirius XM Install Kit for Factory Honda, Acura Vehicles