How do I bind a Field to a lower XPath element that the default? For instance, I have XML with a structure similar to this

<Family>
  <Persons>
    <Person>
      <FirstName>Bob</FirstName>
      <LastName>Smith</LastName>
      <Friends>
        <Friend>
           <FriendName>Ralph</FriendName>
           <FriendAge>22</FriendAge>
        </Friend>
      </Friends>
    </Person
   .etc
   .etc
 </Persons>
</Family>

In the XAML, I have a DataProvider, e.g.
<XmlDataProvider x:Key="FamilyData" XPath="/Family" Source="Family.xml"/>

and a grid bound to it, e.g.
<igDP:XamDataGrid x:Name="_xamDataGrid" DataSource="{Binding Source={StaticResource FamilyData}, XPath=//Person}">

Fields are defined, e.g.
<igDP:Field Name="FirstName" Label="First Name"/>
<igDP:Field Name="LastName" Label="Last Name"/>
<igDP:Field Name="???" Label="Friend"/>

I want the third Field bound to the relative XPath Friends/Friend/FriendName. How do I do this?

Thanks.
Greg