Adding CHA as a stat is as simple as writing it onto your character sheet and making an extra 4d6 roll during character generation.
The skill bonus can than be used in place of your MIND bonus for any interactions with intelligent creatures.

Bluffing past a guard – sub+CHA
Intimidating the barbarian leader with a show of might – phys+CHA
Convincing the elven court that you mean no harm to their forest – com+CHA
Haggling down a merchant – com+CHA

You can still simulate charismatic leaders by having non-magi with a high MIND stat.