I assume you mean a macro. If you want it to always be available,
you should save it in your Personal.xls workbook. I can tell you how to add  a
custom button  to a regular toolbar  in Excel 2003 (not sure how different the
steps are for 2007):

At the right end of all the built-in toolbars  there is a small  down arrow with
the tooltip "Toolbar Options". On the toolbar you want to modify, click that
down arrow. On the menu  that pops up, select "Add or Remove Buttons", then
"Customize..." The Customize dialog appears. On the Commands tab, scroll down
through the Categories and click on Macros. You will see "Custom Button" (next
to a smiley face) in the Commands box. Drag the smiley face icon onto the
toolbar where you want it to appear and release it. Right-click on the new
toolbar item and a menu appears. Assign the macro to run when this button is
clicked, change the button image, give the button a name (used for its tooltip),
etc. Finally, click Close on the Customize dialog.