Fiat and Alfa shared a lot for a long time, the PCD and center bore should match but I'm not sure about the offset, which is where spacers come into the equation.
From what I can see, your steelies have an offset of ET32, the Alfa wheels come in varying offsets from ET37.5 (for the 15x6j wheels), ET40 (16x6j) and ET41.5 (16x6.5j).
So you'd want 5, 8 and 10mm spacers respectively depending on which you go for.
You may also need longer bolts, but you won't know how much longer until you have the wheels in front of you.
To find out what length of bolts you need
Take off one of your current wheels. Measure the length of the shank (the threaded part) of one of the bolts. Put the bolt through the wheel and measure how much of the shank protrudes from the back side.
Now, with the same bolt, put it through the new wheel+spacer and measure how much protrudes.
Take the difference and either add or subtract (I suspect add, as they'll probably need to be longer rather than shorter) to the original shank length to get the new bolt length needed.