I locked the thread because of the statement in the Roadmap ("requests of these features will be ignored/deleted").
A reason why this is not possible yet can be found here (yes, I found it using the Search feature so please do so as well next time 
).
By the way, in case you didn't know, you can view/edit/add/remove additional ID3v2 and APEv2 frames by pressing the key combination [ALT+T].