I have a rod that would be better if the handle were longer.  Can they be lengthened?

Yes, but be leary as many grips are already longer than many elite anglers like on factory rods. Balance is nice, but if the grip length now affects presentation, or your rod's now snagging your clothing, or rain gear your losing fish.

I'D also think of ways to make the tip lighter, more sensitivity and rod performance can be found here. Some of the guides being used by custom builders are 1/6 the weight of most guides found on production rods. These guides are also easy to find, buy, and no more expensive than the guides used now.

I found a rod maker who lengthened the grip by 2 inches and balanced the rod to use with a Curado reel.  I really like it now.