Cannot derive from sealed type Pin. Xamarin forms google maps

I am currently trying to add custom properties to my pin class, and I have to use xamarin.forms.google.maps because of its "Selected pin" event.

public class Pin : Xamarin.Forms.GoogleMaps.Pin // cannot derive from a sealed type "pin".
{
public string CustomProperty { get; set; }
public string CustomProperty2 { get; set; }
public string CustomProperty3 { get; set; }
public string CustomProperty4 { get; set; }

}

So how can I add custom properties? Can I access the nuget package code and unseal it?