Yes, the hub is identical
Same 5x108 PCD and same 65.1 mm centre bore: wheels from Peugeot 408 bolt on and centre on Citroen C6 without rings. You still need to check the offset and tyres, detailed below.
≈: width estimated from the original tyre size.
Wheel by wheel: offset and tyres
Each original wheel from Peugeot 408 is compared with the one from Citroen C6 of the same diameter: how much closer the wheel gets to the shock absorber or how far it sticks out of the wheel arch, and whether the tyre keeps the same overall diameter.
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7Jx17 ET42 · 215/65R17 — Check on the car
Compared with the original 7Jx17 ET32 on Citroen C6: shock absorber side +10 mm, wheel arch side −10 mm, tyre diameter +4.7 % : fit it with the original tyre size of Citroen C6 (225/55R17), not that of Peugeot 408.
Check in detail
Before fitting the wheels
An identical hub is not enough: offset, width and diameter change the position of the wheel, and larger calipers can touch a smaller wheel. Original wheels often use bolts or nuts with a specific seat: keep the fixings intended for the wheel, with the thread of Citroen C6 (M12x1.25), tightened to the torque shown on its vehicle page. Finally, check that the size is among the approved fitments for your vehicle. This comparison is for guidance only: have the fitting approved by a professional.
And the other way round: wheels from Citroen C6 on Peugeot 408?