prueba esto , no se si sea lo que quieres. le puse un menu item "Incrustar_formulario" puedes usar un boton comun tambien :/...
Private Declare Function SetParent Lib "user32" (ByVal hWndChild As Long, ByVal hWndParent As Long) As Long
Private Sub Form_Load()
Form2.Show
End Sub
Private Sub Incrustar_formulario_Click()
Call SetParent(Form2.hWnd, Me.hWnd)
End Sub