You could use a Globalpass

Or you look for cheap fares on sbb.ch for Zürich Flughafen to Milano

And travel within Italy with a Italy One Countrypass (offer of Eurail) or a Trenitaliapass (offer of Trenitalia)

IF this is the only trip In IT, then simply buy a add-on single Chiasso (=border point) to Mil. Do not waste a passday for this-its just 50 kms. Its even cheaper when you detrain there and use local regional train-less as 10€. 

Thank you for the great answer.  

The trip from Zürich Airport to the closest Italian city, Chiasso, is 76.00 Swiss Franc one way without discount in 2nd class. I would suggest that you buy this ticket separately and then buy a one country Interrailpass for Italy. From Chiasso to Milan you will then use the Interrailpass.