I cannot find a link to information about luggage storage. I have looked on the Malpensa website. Any help would be appreciated.
You could contact them via email. I would be very surprised to find luggage storage at the airport, part of the security measures include moving luggage and packages out of the airports.
There is a luggage storage at the Milan Centrale train station. 13 euros a day with a maximum of 5 days. That could be an option since I can't find one at the airport.