[REQ] Substitute generated class by custom class extending this class

Is your feature request related to a problem? Please describe.

We would like to add some behaviors to the generated DTO, but we can't do it in the generated classes (new behaviors are class specific, so can't be global).

Describe the solution you'd like

A way to substitute a class by a custom from our project. Here is an example.

Generated class

package com.myapp.generated;

public class GeneratedClasseDTO {
 ...
}

Custom class (hand written) :

package com.myapp.custom;

import com.myapp.generated.GeneratedClasseDTO;

public class CustomGeneratedClasseDTO extends GeneratedClasseDTO  {
 ...
}

Another generated class

package com.myapp.generated;

public class AnotherGeneratedClasseDTO {

    // here the generation doesn't use directly the generated GeneratedClasseDTO class, but our custom class
    private CustomGeneratedClasseDTO generatedClasseDTO ;
}

<configuration>
    <substitutes>
        <substitute>
            <generated>com.myapp.generated.GeneratedClasseDTO</generated>
            <custom>com.myapp.custom.CustomGeneratedClasseDTO</custom>
        </substitute>
    </substitutes>

Describe alternatives you've considered

We use services/helpers, but we don't like it.

The issue names no source files, tests, or entry points. Start by reviewing the requested XML substitution configuration and the generated Java DTO relationship; done means a configured custom subclass is used wherever the generated class would otherwise be referenced.