There are leaders, and there are charismatic leaders. How does a leader become more charismatic?
I believe that "charismatic" isn't something you call yourself - It's something your audience / team / followers calls you.
In my experience charismatic people do tend to be extraordinarily passionate people.
So perhaps to become more charismatic - be more passionate.
First thing to remember is that you don't need to be a 'charismatic' leader to be successful.
I believe that when you speak from the heart about something you are passionate about and your goal is to help someone other than yourself (eg. employees, customers, community, etc...) your true charisma which we all possess will come to light.
I hope this helps