Adding cell format options to Custom list

The templates Book.xltx and Sheet.xltx do not exist when you install Excel. You have to create them yourself, as described in my previous reply. You have to do that only once.

When you have done that, Excel will use Book.xltx as template when you create a new workbook by pressing Ctrl+N or by clicking the New button on the Quick Access Toolbar, and it will use Sheet.xltx as template when you insert a new worksheet.

I don't know of a way to make custom number formatting available to already existing workbooks, but you can make them available in all new workbooks:

• Create a blank new workbook.
• Create the custom number formats that you want.
• Make sure that you clear both the contents and the formatting of the cells that you used to define the custom formats.
• Set up other aspects of the workbook the way you want (number of worksheets, default headers and/or footers, cell styles, ...)
• Save the workbook as an Excel template named Book.xltx (this name is obligatory; it is language-dependent, in the German version of Excel it would be Buch.xltx) in your Excel startup folder. The startup folder is
   
  C:\Documents and Settings\<username>\Application Data\Microsoft\Excel\XLSTART
   
• Warning: the Application Data folder is a hidden folder, so you must show hidden files and folders to see it.
• Also create a copy of this template with just one worksheet, and save it as Sheet.xltx in the same folder.