You can, however try to clone the existing renderer as defined by the service and modify the symbol base on your preference. Your custom symbol will then have VisualStates for MouseOver or Selected.
void l_Initialized(object sender, EventArgs e)
{
FeatureLayer l = sender as FeatureLayer;
if (l.Renderer is UniqueValueRenderer)
{
var current = l.Renderer as UniqueValueRenderer;
var renderer = new UniqueValueRenderer() { Attribute = current.Attribute };
foreach (var info in current.Infos)
{
var newInfo = new UniqueValueInfo()
{
Description = info.Description,
Label = info.Label,
Value = info.Value,
//TODO: clone existing symbol or overwrite with new symbol
Symbol = this.LayoutRoot.Resources["MyCustomSymbol"] };
renderer.Infos.Add(newInfo);
}
l.Renderer = renderer;
}
}
