I have a spreadsheet with a customer number on line 1, with additional data
about this customer on lines 2-6 (customer column on these lines is blank).
I then have a new customer number on line 7, with additional data about this
second customer on lines 8-12 (custom column on these lines is blank). This
spreadsheet has thousands of such entries.

I am hoping to find a command that will allow me to fill in the custom
number from line 1 down until I find the next custom number already filled;
then take the new customer number and fill it down until I find the next
customer number, etc. Do you know of a command that will help me?