As far as I know there is no dedicated fast track line. To be fair, if such a service exists in MXP it's a bit of a waste of money because the process is rather simple and staightforward.

Some enterprising mind has even done a Google streetview of the route from Satellite C to immigration and the arrival hall. Mind you that customs, in Italy (and much of Europe) is very simple. Sometimes the customs officers stop you, sometimes they don't.

https://www.google.com/maps/@45.6308...7i13312!8i6656