First off the S5 should have memory modules even if you don't have the buttons on the door. This has been standard on most of the A5 coupes since the start of the B8's.
As far as retrofitting into the Sport-back..
Short answer is yes it can be done. I've done it on a B9 A4 which is just about the same vehicle.
Long Answer:
You will need the
1) door trim
2) memory switch
3) Memory Switch plug
4) 5 Pins I think for the memory plug and 3 for the door module plug. 2 others tap in.
5) 2 wiring harnesses for the seat (as both the back and bottom are different)
6) Can bus wire to run to the gateway from seat (comfort bus)
7) 2 pins for the bottom existing plug for the can bus wire
8) Module Holder
9) Module
10) Seat switch
11) for coding
extended info...
The modules vary, but from what I can tell you need a module with no revision. as in no letter at the end. I know they make different modules based on years but I used a 4M0 module on car that should have 4M4 and it worked perfect.
You need a used module.. Good Luck.. Took me forever to find one but it may be because of where I'm living now. Not everyone will ship here. You can get a new module but you need to find someone with a comparable car to take it into the dealer and get it loaded with parameters.
Now I don't know the last for sure but it's a fairly good bet since Audi and VW are using parameters for just about all their modules now a days. There is no online protection for seat modules. No dealer trip needed.
Seat harnesses.
You can cheat here. The original harnesses are not that expensive, however. If you want to save some money and pain of replacing the rear seat harness you can simply cut the 1 plug that goes up into the back of the seat from your original harness and use it to replace the 2 plugs the new harness uses.
Will require some cutting and soldering to do properly. You will be left with one extra wire on the old plug that that the new one doesn't use. If you choose to use a new harness no big deal. Just make sure you have lots of different sized trim tools and patients to get the old harness removed and the new one in its place.
Good Luck.
P.S. Coding.. If you get a used module that is opposite of your car I.E. RHD, LHD. It's fine but the coding is different and you will need to re code.
If you want Easy entry and exit, enable it in the module and make sure you enable electric steering column in module 9. That will bring up the menu entry in the and make the feature work. It will not cause any errors.
Best Regards,
David