Ummm...That's an interesting problem.
Just copy the existing Goal Seek... menu item.
Select Tools | Customize... Then, while in customize mode, click on the Tools menu again. Llet go of the mouse and the Tools menu will remain visible. Move down to the Goal Seek... item, hold down the CTRL key, click and drag off the Tools menu. If you do this correctly, you should see a '+' sign next to the mouse. Put the copied Goal Seek... menu item on the commandbar of your choice.