Hi there!
With this plugin you can insert this extra parameter on the class note tags. Either by using the Templates or by writting it down directly. That information is on the help file. Or your intention is not use the default plugin way(note tag or plugin parameter) to add this extra parameter? if yes, then why?
To retrieve any data from a data class object use the script call:
$dataClasses[CLASS ID] → Replace CLASS ID with the id of the class you want.
Usually, when people want to do something Dynamic with the Custom parameters, they also use another plugin of mine:
https://hakuenstudio.itch.io/eli-dynamic-parameters-for-rpg-maker
Hope it helps!