public string ExpandStateFieldName { get; set; }

Public Property ExpandStateFieldName As String

<dxg:TreeViewControl ItemsSource="{Binding EmployeeDepartments}" 
                     ChildNodesPath="Employees" 
                     TreeViewFieldName="Name"
                     ExpandStateFieldName="Expanded"/>

public class EmployeeDepartment {
    public string Name { get; set; }
    public ObservableCollection<Employee> Employees { get; }
    public bool Expanded { get; set; }
}

Public Class EmployeeDepartment
    Public Property Name As String
    Public ReadOnly Property Employees As ObservableCollection(Of Employee)
    Public Property Expanded As Boolean
End Class