If the fixture you have runs 18w bulbs then you can only ever run 18w tubes in it, you can run a 40w bulb on a 18w starter. The only way to increase the WPG is to add more tubes or to change the existing starter for a higher wattage fixture such a power compact or such like, but you'll have to replace all the electronics to do that.
The simplest way is to add another tube, I've yet to see a hood you cant add more light to.
Sam