Change Name in Comment

How can I change username variable in comment form?
I have my own user table, so I want to synchronize the comment field with it.
In what file I can change the value of the username variable?
thanks

Jot uses default user data (web_user_attributes table).
You can use PHx modifier to get custom data.

can you explain me more about PHx modifier to change custom data?

Read about PHx modifiers: http://wiki.modxcms.com/index.php/PHx#Modifiers
And some modifier codes as sample: http://wiki.modxcms.com/index.php/PHx/CustomModifiers