You need to create a partial class file, to be preserved across code generations.
And if the custom property is common among all your DTO classes, then you can create a base class to have the extra property(s), and go to** Project->Settings->Derived Mode**l, select your derived model, click on DTO Class Model, and use the setting Root Derived Element base class name. Then all your DTOs will be extending that base class that you should write with your custom stuff.