Actually, you can't "fill it in"... but you CAN thicken the lower biceps (but not lengthen it) and make it appear to be filled in.
Do preacher curls with a close (narrow) grip. Make sure you lock that body in upright and don't lean. Forget about the amount of weight and focus on the amount of stress you are driving into that lower area. (you should be using a lower weight anyway because with heavy weight, people are prone to popping/snatching the weight at the bottom of the rep to get it started back up... BAD move). Full reps, all the way down, slight pause, all the way up.
Do curls off the vertical side of the preacher bench. (dumbells)
Hammer curls add enhancements as do reverse barbell curls.