It shouldn't hurt a thing unless, of course, you are transmitting on that antenna. Adding length may or may not help a thing if you add length to it. The only time that I have had better luck by adding length is while I was monitoring low band.
If you are transmitting on an antenna, then the antenna would surely have to be resonant on the frequency on which you are transmitting. Some approximate (really rough) figures for 1/4 wave antenna lengths are listed below.
VHF Low band (30-50 MHz) - 6'
VHF High band (150-174 MHz ) - 18"
UHF (450-470 MHz) - 6"
800MHz - 3"
If you want to add gain to antenna, then you should pull out the Amateur Radio Handbook or buy an antenna book...