How to automatically add letters to end of cell?
You can use this formula:
="ZL24N65"&CHAR(SEQUENCE(26,1,65))
SEQUENCE will create a list of integers 26 items long, starting at 65. Passing that list into CHAR will create a list that represents the capitalized alphabet. We are then prepending the leading characters to that list.
If you don't know your data in A1, you can use:
=LEFT(A1,LEN(A1)-1)&CHAR(CODE(RIGHT(A1))+1)
As I mentioned in this cross-post (though on a different website)