App.xaml

<Application.Resources>
  <loaders:XmlResourceLoader x:Key="GlobalProductClassifications" Path="data/external/Custom_GPCList.xml" Type="{x:Type gs1:Schema}" /> 
</Application.Resources>

View.xaml

<ListBox ItemsSource="{Binding Segments, Source={StaticResource GlobalProductClassifications}}" />

XmlResourceLoader.cs

public class XmlResourceLoader : MarkupExtension
{
    public string Path { get; set; }

    public Type Type { get; set; }

    public object Value { get; private set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        if (this.Value != null)
            return this.Value;

        try
        {
            using (var stream = File.OpenText(this.Path))
                this.Value = new XmlSerializer(this.Type).Deserialize(stream);
        }
        catch (Exception e)
        {
            Log.Error(e, "Error while deserializing '{path'} into '{type}'.", this.Path, this.Type);
        }
        return this.Value;
    }
}