You can (freely) add custom privileges to the system via the priv table.
Or you can use the delivered function:
SELECT xt.add_priv(privilege_name, priv_description, module, privilege_group, NULL);
Name and Description should be obvious. Module is to define things like 'Sales', 'Manufacturing'. Look in the priv table for examples. Privilege Group is to define where in the assignment screen the privilege will be found. I would suggest creating a new group for your custom privilege.