Just an update to this - my recent experience is that no Apple Store I have questioned is able (or prepared?) to upgrade RAM on an existing iMac Pro. They will replace dead RAM (probably with a complete machine replacement) but they absolutely won't entertain an upgrade.
One went so far as to say that they don't even have model numbers for suitable SIMMs (yes, SIMMs... doesn't exactly inspire confidence that they are talking about memory modules from two decades ago...)
Oh good grief. You apparently talked to an idiot. They should know exactly how to look for those part numbers, they're readily available in the service docs.
RAM part numbers for iMac Pro:
661-08891, 8GB, DDR4 ECC, 2666MHz
661-08892, 16GB, DDR4 ECC, 2666MHz
661-08893, 32GB, DDR4 ECC, 2666MHz
661-12559, 64GB, DDR4 ECC, 2666MHz
That said, don't even think about taking your iMac Pro to an Apple Store for an upgrade. The OEM RAM prices are absolutely insane, they'll charge you more for one 32GB DIMM than you'd pay on Amazon for a set of four, and that doesn't include labor. The 64s are close to $1500 each if you buy them from Apple.
You're going to be MUCH better off going to either an unauthorized service provider or an authorized service provider who is willing to break the rules and use non-Apple RAM. (And since Apple doesn't actually make any RAM at all...)
And of course saying it can only be done by Apple or AASPs is a lie, you can even DIY it (but if you do, be VERY careful, the display is fragile once it's no longer attached to the enclosure, and it's also stupid expensive to replace. The cables that connect it to the logic board are also fragile). See the iFixit teardown to get a good idea how:
https://www.ifixit.com/Teardown/iMac+Pro+Teardown/101807
The VHB strips and VHB cutter are readily available from many sources. Do not screw around and use a knife, the cutter is cheap and is designed not to damage anything. Oh, and get the foam service stand that holds the unit in place, it'll make your life easier. A strip of shipping tape across the bottom of the display to hold it in place is your friend too. The display cables are not long enough to allow you to move the display very far, they must be disconnected with it open just a bit, you won't have much room to get your hand in there. It's not the easiest disassembly ever, so be very careful and take your time.
Oh, and be sure you're static strapped up, and DO NOT open it with it plugged in, parts are live at full line voltage if it's plugged in.