The FindResult object contains a feature object with all its attributes. So you can extend your FeatureItem to store other attributes:
.OtherAttribute = .elem.Feature.Attributes["AttributeName"]
or even to store the whole Feature:
.Feature= .elem.Feature
That will be easier if you store the Feature in your FeatureItem. So (as in the sample) from the SelectedItem, you can retrieve the Feature and show it in a graphics layer.
I also need the legend dymbol for the layer. The image I have in the feature class is to store the legend symbol for the layer. How we can access that information?
Now I need to add a MouseEnter/Leave event to the Listbox that has the result, but this listbox is defined inside the DataTemplate. How and where I could add those event?
<DataTemplate x:Key="FeatureItemDataTemplate"> <Grid MouseEnter="Grid_MouseEnter"> ........
If you have included a legend control in your application, you can use it to get the swatches of the sublayers (look at this thread http://forums.arcgis.com/threads/23147-populating-sublayers-on-a-listbox-along-with-the-symbols) else you could call QueryLegendInfos of the service by yourself.
Why not just hook up your handler in XAML?<DataTemplate x:Key="FeatureItemDataTemplate"> <Grid MouseEnter="Grid_MouseEnter"> ........