I’ve looked at doing the same on my S5 (B9) and from what I’ve seen online it isn’t possible to fit an OEM Grille without some major modification. Seems to be a lot easier and cheaper to buy a Chinese replica.
Only issue is they don’t seem to have managed to get the front camera sorted which is an issue having it spec’d on my car. Normal front parking sensors are fine though.
My earnest advice is don't do it.
Having had an RS with the grill you covet and now having an S4 which has a more sensible grill I do not miss the RS grill one little bit.
It is extremely difficult to clean and keep clean and I am glad to see the back of it.
The other thought is why have bits and pieces of an RS on a non-RS vehicle - bit of an insult to the marque IMHO?
The S4 and S5, are great designs in their own right.