I guess you are talking about wxSmith here.
On the right side of the wxSmith "editor" you should have some buttons, id you place the mousepointer over theme a tooltip should be shown.
Click the fourth button ("Insert new widgets after current selection"), select the sizer,that should be before the new one in the "Resources"-tab of the "Management"-pane (or directly with the mouse), then click on the appropriate sizer in the "Layoout"-tab of the wxSmith "editor" and it should appear where you want it.