Hi, 

So are you just looking to put something over the entire header of the XamGrid? Or you want to have a Header over some columns, not all of them?

If you're just looking to have one header bar over the entire grid, just wrap your XamGrid in a Grid panel.

<Grid>
     <Grid.RowDefinitions>
            <RowDefinition Height = 20/>
           <RowDefintion/>
      </Grid.RowDefintions>

      <TextBlock Grid.Row = 0 Header="Title"/>
     <ig:XamGrid Grid.Row = 1/>

</grid/> 

If its the Latter you're looking for, then this is actually a feature will be coming out in the 10.3 release. 

Hope this helps,

-SteveZ