I have a control that accepts a string, but when it displays, I want to add "Hello" to this string directly in xaml, how can I do this?
You can bind to a property and use StringFormat:
<TextBlock Text="{Binding Path=UserName, StringFormat=’Hello \{0\} ’}"/>
More information can be found here: http://www.designersilverlight.com/2010/05/28/silverlight-4-binding-and-stringformat-in-xaml/