Here's a sample of customizing BorderBrush. It is similar to customizing the Fill.
<esri:SimpleFillSymbol x:Name="DrawFillSymbol" Fill="Black" BorderThickness="10">
<esri:SimpleFillSymbol.BorderBrush>
<LinearGradientBrush x:Name="MyBrush" StartPoint="0,0" EndPoint="1,1">
<GradientStop Color="Yellow" Offset="0.0" />
<GradientStop Color="Red" Offset="0.25" />
<GradientStop Color="Blue" Offset="0.75" />
<GradientStop Color="LimeGreen" Offset="1.0" />
</LinearGradientBrush>
</esri:SimpleFillSymbol.BorderBrush>
</esri:SimpleFillSymbol>