I'm not sure it's possible (well, it was impossible for me!).
But one alternative is to define a style that uses that format.
Format|Style
type a new name in the Style name box (say myDateStyle)
and click the modify button
Click on the number tab and use a custom format of mmmm d, yyyy
Then you can apply that style to any cell you want.
But styles live in workbooks.
So I'd create a workbook template named book.xlt with that style. Store that
template in your XLStart folder.
Now every workbook that you create by clicking on the New icon on the Standard
toolbar will inherit this style.
And to make it easy to change styles, you could add an icon to your favorite
toolbar.
Tools|customize|commands tab
Format category
There's a Style... icon you can drag that will show the dialog.
But there's a Style icon with a dropdown box that you can drag to your favorite
toolbar.
Then you can just select the cell, use the dropdown, choose myDateStyle and
you're done.
