You'ld have to replace the entire fixture, there are double strips available, there are also Compact Fluorescent available, but it depends on how far you want to go into plant keeping.. almost every plant benefits from Co2
addition @ more then 40w of T12 lighting, it's pretty much mandatory. (the sticky has a lot of info on this stuff)..
Double T8 (smaller bulb, more efficient) can be picked up pretty cheap from Petsmart or the ilk. (about 35$) and will probably be a decent easy start.. You may also want to read up on the ferts part of the sticky (might say dosing..) since you will probably need to add a bit of Potassium every now and then (at water changes probably)..
120 Gal SW, 150lbs Prem. Fiji, 23Gal Sump, 2x250w 15K MH, 4x65w Actinic
3 x Halichoeres Chrysus (1 surfed), 3 x Nemateleotris magnifica, 1 x Centropyge bicolor, 11 x Scarlet Hermits, 6 x Zebra Hermits, 40 x Astrea Snails, 6 x Nassarius Snails, 3 x Cerith Snails