Use this method:
[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern int SetWindowLong(System.Runtime.InteropServices.HandleRef hWnd, int nIndex, int dwNewLong);
/// <summary>
/// sets the owner of a System.Windows.Forms.Form to a System.Windows.Window
/// </summary>
/// <param name="form"></param>
/// <param name="owner"></param>
public static void SetOwner(System.Windows.Forms.Form form, System.Windows.Window owner)
{
System.Windows.Interop.WindowInteropHelper helper = new System.Windows.Interop.WindowInteropHelper(owner);
SetWindowLong(new System.Runtime.InteropServices.HandleRef(form, form.Handle), -8, helper.Handle.ToInt32());
}