/$/,/g add to end of every line

:%s/$/,/ will add a comma to, the, end, of, every line

(vim) do a subset of lines instead of the whole file, specify them in place of the %.

One way is to do a visual select and then type the :. It will fill in :'<,'> for you, then you type the rest of it (Notice you only need to add s/$/,/)

:'<,'>s/$/,/