Try this instead:

Text = "\xE10F" 

You should be able to use the Character Map (charmap.exe) application to copy and paste characters from Segoe MDL2 Assets to Illustrator.

If I understand you correctly, You want to have more symbols in your app than the list you mentioned provides.

Yes, you can do that. If you want to keep using SymbolIcon, then you have to either set it's Symbol in code behind or use x:Bind.

Setting the symbol in code behind:

If your XAML looks like this:

<SymbolIcon x:Name="MySymbolIcon" />

then your code behind is:

MySymbolIcon.Symbol = (Symbol)0xE156;

Or, using x:Bind :

If you have a Symbol in you code behind like this:

Symbol Avatar = (Symbol)0xE156;

then you can x:Bind to it in the XAML like this:

<SymbolIcon Symbol="{x:Bind Avatar}" />

And now you may wonder, where will you find these creepy looking hex codes? Well, there's an app Character Map UWP which will provide you a nice list like this:

Or, Using FontIcon :

You see the Codes at the bottom right corner of the image I included? There's code for FontIcon too. Just copy paste it.

Hope that helps.

It is possible, though a bit complicated.

Instead of the GeometryDrawing with a VisualBrush you may perhaps also use a GlyphRunDrawing. However, that looks even more complicated.

<Image Stretch="None">
    <Image.Source>
        <DrawingImage>
            <DrawingImage.Drawing>
                <GeometryDrawing>
                    <GeometryDrawing.Brush>
                        <VisualBrush Stretch="Uniform">
                            <VisualBrush.Visual>
                                <TextBlock FontFamily="Segoe MDL2 Assets"
                                           Text="&#xE8C6;"/>
                            </VisualBrush.Visual>
                        </VisualBrush>
                    </GeometryDrawing.Brush>
                    <GeometryDrawing.Geometry>
                        <RectangleGeometry Rect="0,0,32,32"/>
                    </GeometryDrawing.Geometry>
                </GeometryDrawing>
            </DrawingImage.Drawing>
        </DrawingImage>
    </Image.Source>
</Image>

For easier reuse, you may create a StaticResourceExtension like this:

[MarkupExtensionReturnType(typeof(DrawingImage))]
public class IconImageExtension : StaticResourceExtension
{
    private static readonly FontFamily fontFamily
        = new FontFamily("Segoe MDL2 Assets");

    public int SymbolCode { get; set; }

    public double SymbolSize { get; set; } = 16;

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        var textBlock = new TextBlock
        {
            FontFamily = fontFamily,
            Text = char.ConvertFromUtf32(SymbolCode)
        };

        var brush = new VisualBrush
        {
            Visual = textBlock,
            Stretch = Stretch.Uniform
        };

        var drawing = new GeometryDrawing
        {
            Brush = brush,
            Geometry = new RectangleGeometry(
                new Rect(0, 0, SymbolSize, SymbolSize))
        };

        return new DrawingImage(drawing);
    }
}

and use it like this:

<Image Stretch="None"
       Source="{local:IconImage SymbolSize=32, SymbolCode=0xE8C6}"/>