You can sort the view of the collection rather that sorting the collection itself:
// xmlns:scm="clr-namespace:System.ComponentModel;assembly=WindowsBase"
<myView.Resources>
<CollectionViewSource x:Key="ItemListViewSource" Source="{Binding Itemlist}">
<CollectionViewSource.SortDescriptions>
<scm:SortDescription PropertyName="SortingProperty" />
</CollectionViewSource.SortDescriptions>
</CollectionViewSource>
</myView.Resources>
And then you can use the CollectionViewSource as ItemSource:
ItemsSource="{Binding Source={StaticResource ItemListViewSource}}"