Hi,
This should be quit simple using our special API for collections WillReturnCollectionValuesOf :)
In the example bellow once SPList.Items is called it will return a collection with one SPListItem.
SPList fakeList = Isolate.Fake.Instance<SPList>();
Isolate.WhenCalled(() => fakeList.Items).WillReturnCollectionValuesOf( new List<SPListItem>
{
Isolate.Fake.Instance<SPListItem>()
});
Please let me know if it helps.