You could use a template workbook (like Jacob suggests) for new workbooks.
But that won't help you for existing sheets in existing workbooks. And the
book.xlt workbook won't help for adding sheets to any workbook.
You could create a second template workbook and name it sheet.xlt and excel
would use this for new sheets. (Save it in the same XLStart folder that Jacob
mentioned.)
But I wouldn't do this. I'd end up having to remove it from worksheets where I
don't need it.
Instead I'd start a new workbook
record a macro when I added the footer to a worksheet in that workbook.
Stop recording
Save this file with a nice name: AddFootersToActivesheet.xls
And reopen this workbook whenever I needed to do this.
Then use Tools|macro|macros (or alt-F8) to run the macro when I needed it.