Import separate XAML as ResourceDictionary and assign x:Key

I downloaded the Visual Studio Image Library and I see that the included icons are available in vector format in the form of .XAML files. Here is an example:

Add_16x.xaml

<!-- This file was generated by the AiToXaml tool.-->
<!-- Tool Version: 14.0.22307.0 -->
<Viewbox Width="16" Height="16" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
  <Rectangle Width="16" Height="16">
    <Rectangle.Fill>
      <DrawingBrush>
        <DrawingBrush.Drawing>
          <DrawingGroup>
            <DrawingGroup.Children>
              <GeometryDrawing Brush="#00FFFFFF" Geometry="F1M16,16L0,16 0,0 16,0z" />
              <GeometryDrawing Brush="#FFF6F6F6" Geometry="F1M5.0004,-0.000199999999999534L5.0004,4.9998 0.000399999999999956,4.9998 0.000399999999999956,10.9998 5.0004,10.9998 5.0004,15.9998 10.9994,15.9998 10.9994,10.9998 16.0004,10.9998 16.0004,4.9998 10.9994,4.9998 10.9994,-0.000199999999999534z" />
              <GeometryDrawing Brush="#FF388A34" Geometry="F1M10,6L15,6 15,10 10,10 10,15 6,15 6,10 1,10 1,6 6,6 6,1 10,1z" />
            </DrawingGroup.Children>
          </DrawingGroup>
        </DrawingBrush.Drawing>
      </DrawingBrush>
    </Rectangle.Fill>
  </Rectangle>
</Viewbox>

I want to put all the icon xaml files I want to use in my application source in a folder called "Icons" and then be able to have a file called IconDictionary.xaml somewhere else that defines a ResourceDictionary holding a MergedDictionaries and inside this MergedDictionaries I want to be able to include the icon.xaml files somehow and assign them an x:Key property so I can refer to them throughout my application as static resources.

Is it possible to use this icon.xaml file without modifying the .xaml file itself?

I would like to be able to just leave them as they are but unfortunately it seems like I have no choice but to either copy their contents into my IconDictionary.xaml or edit the individual .xaml files and surround them in a ResourceDictionary with a x:Key that I could add to my MergedDictionaries.

UPDATE

Here is what I would like my IconDictionary.xaml to look like for clarity:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <DataTemplate x:Key="Add_16x">
        <!-- Import Viewbox from Add_16x.xaml here... -->
    </DataTemplate>

</ResourceDictionary>