Yes, the hub is identical
Same 5x115 PCD and same 70.3 mm centre bore: wheels from Opel Cascada bolt on and centre on Cadillac ATS 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 Opel Cascada is compared with the one from Cadillac ATS 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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≈7.5Jx17 ET41 · 235/55R17 — Check on the car
Compared with the original ≈7Jx17 ET42 on Cadillac ATS: shock absorber side +5.4 mm, wheel arch side +7.4 mm, tyre diameter +8.8 % : fit it with the original tyre size of Cadillac ATS (225/45R17), not that of Opel Cascada.
Comparison for guidance only (estimated width). 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 Cadillac ATS (Ecrous - 12x1.50), 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 Cadillac ATS on Opel Cascada?