Create a .desktop file containing the following as below


[Desktop Entry]
Name=
Exec=
Terminal=false
Hidden=true
Type=Application
Icon=/path-to-icon/icon.svg



Where /path-to-icon/icon.svg is the icon you want to
use for your separator. Then add it to the dock.


There are 4 desktop files because you may only
use 1 desktop files per separator. As well as
we made each a different distance on the horizon.

BTW You will need to copy the desktop files to 
anywhere inside your home directory drop them, then
slide them to the dock (after dropping them you now own them
not the root user :-)