Possibly. Your bulbs right now are 15watts each, 30watts total. You'll be hard pressed to get decent growth from even the lowest of low-light plants.
You could retrofit your 2 existing hoods with power compact fluorescents. Check out
www.ahsupply.com and their 55watt bright kits. That'll put you at closer to 2wpg though, which starts getting into
CO2 territory, and a little more maintenance to keep everything happy.
Another option would be a 48" fixture for
NO bulbs which would give 40watts total. You could add that to your current bulbs, and have 70watts total, which would still be low light, no
CO2 needed, pretty easy going on the maintenance.
You could also overdrive your current fixtures, by wiring in bigger ballasts and tricking them into pushing more power to the bulbs. The bulbs have to be replaced every 4 months though.
You have a few options, but all of them are going to cost some $$...figure out what fits your budget and goals the best.