Is it possible for the dealer to add on after the fact the oem rear seat entertainment system?
Yes it is possible, I just did it myself.
The installation is not a big deal for anyone handy with tools but you gain the OEM warranty of you get the dealership to install.
I paid a fraction of what Ford wants for the Dual Headrest DVD package but I would recommend some of the newer style kits with iPads or tablet of your choice. The original ones were hokey yes, but the newer ones are much more integrated and gives you the ability to remove them.
If you do decide to go OEM don't forget to download the Ford Play App, it adds some additional functionality.
The new ones allow for interactive games and anything you do on the smartphone can be pushed up. I like the idea also of integrating the headrests with the unlimited data plan in the vehicles 4G hotspot. You can watch live programs and surf the web on these new ones if you pay the unlimited data plan.
I can send you the installation document if you decide to go that way.