I do not think that this is an existing feature in neovim, however this seems to work
:%s/\(.*\)\zs\s*$/\=repeat(' ', 15 - len(submatch(1)))
Change 15
to the column desired. You could probably create a function where you pass the column number you want so that you dont have to type this string all the time.
Use by typing in Normal mode
:PadColumns 20
. This will add spaces after the line or selected lines to the column you specify (in this case, 20).You could probably improve this by getting the length of the longest line and so you dont need to specify the specific column to add spaces to (20), and instead just add say 5 spaces after longest line for all lines.