Charisma score does not affect druid or necromancer charm ability. Negative CHA does not increase a necromancer's charm chance. The best way to increase your charm duration with either DRU, SHM, or NEC is based on level difference and the mob's Magic Resist, so if you have an Enchanter in your group, have them cast Tash. Magicians and Shamen can malo to lower MR.
If you want to search the Allakhazam patch archive for evidence of charisma affecting druid charm, you can check
http://everquest.allakhazam.com/history/patches.html . I couldn't find anything with some cursory searching.