You should be able to save the workbook as an add-in.
With a new workbook, place the Function in a module, the save the
workbook as an Add-in. F12=name the Workbook=select file type as Add-
in.
Close excel and open excel, go to tools,add-ins, and find your new add-
in,if you do not see it hit the browse button, and find it wherever
you saved it.

You can also place the code in your Personal Macro Workbook, this will
make the code available to all workbooks