Why conquer them? The vast majority (The Duchy of Savoy and the Venetian Republic being the only exceptions) tended to march lockstep with Spain, owing to the Army of Italy in Milan. When the Italian Wars were said and done, Spain already had de facto domination over the Italian peninsula. They got the benefits without any of the negatives (such as having to pay for more garrisons and administrative costs).
Ahem. Not under Phillip II, though it is an Iberian superpower.Would it be possible for the Spanish Empire, under the reign of Felipe II, to conquer the Italian city-states, establishing an Ibero-Italian union which later would proceed to become a world superpower?