adding a parameter to a method of factory for persistent class

Hi community,

i need to add a parameter in the method get_persistent of a customized class, which is factory for persistent class. I see that you can do that trough Mapping Assistant, which is available upon creation of class. But if i need to modify now, i get the: 'Co-classes of persistent classes cannot be edited directly'. Is there a way to do that now or these cannot be further edited?