When you add a new attribute to the EO the EOImpl won't be modified as .java files are not customizable. However, this does not mean that EOs are not customizable. When you add a new attribute to the EO via customization then those customizations are recorded in the MDS and getter, setters are generated after customization layer is merged to the base document in the run time. So, adding a new transient attribute to the EO via customization definitely works.
And this is true for VOImpl as well in case of VO customizations.
I would advise you to look into your custimization again or try out with minimal steps. It should work.
Hope this helps