It is possible, my dealership installed the navigation on my non nav GT premium.
I don't remember the price off top, but I know it was the OEM standard navigation system price.How much did you end up paying for that?
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You are correct that a dealer update of the SYNC system will wipe NAV. This goes for all post-OEM navigation upgrades done by any provider at this point. If they (the dealer) ever wants to update SYNC...don't let them and contact me first. I can talk to them about it and any issues.I added the Nav from Ford PI Mods http://www.fordpimods.com/navigation-mft.html
Sent in my APIM (sent it on Monday, had it back on Friday), it works exactly like factory, only issue would be in the APIM were replaced or dealer did a OASIS SYNC update you would loose nav (but it sounds like they'd work with you on the APIM replacement, and MFT update using VCM is a pain so dealer would only do it if you had a MFT complaint.
That's on MyFord touch (2015's), so far nothing for SYNC3 that I've seen.