It is impossible to add only a buff to a unit. However there a few ways that you can simulate doing so.
You can have a dummy unit spawn and cast a spell with the buff you want on the unit. This way it looks like nothing happened except for the unit gaining the buff.
You can create an ability based on the slow tornado aura to create a debuff (red text), set its targets allowed to only self, then just add it to the unit you'd like to have the debuff.
Inversely, if you want to create a buff (green text), just do the same as above, create an ability based on slow tornado aura, just name it's corresponding buff with the following color code:
|cff00ff00your buff name here|r
This will make the text green, so it looks like a buff instead of a debuff.