Ok, I got it.

In your example select col J ( the first one after your table) - Right Click - Format Cells - Border and add a left border in the border " zone" ( or whatever it's called)
Now you can drag any cell from col I without loosing the right border ( which is actually col J's left border)

Apply an equivalent technique if your first column is not col A.

__________________
Using O365 v2503 - Did you know you can thank someone who helped you? Click on the tiny scale in the right upper hand corner of your helper's post