As you indicate yourself, End of Evangelion is a movie. Consequently, it is listed in the movie part of our database and shouldn't be duplicate as a special episode of a series.

The series is locked specifically to avoid this type of duplication.

I understand that The End of Evangelion is correctly listed as a film. However, I would like to emphasize that the second segment of the film, "One More Final: I Need You", is literally titled "Episode Twenty-Six" and serves as a retelling or direct continuation of episode twenty-six of the show.

This is not to duplicate the film, but to recognize that this part has the structure and function of an episode, including official numbering. Many fans and scholars consider End of Evangelion to be episodes twenty-five and twenty-six, and this narrative distinction is important to contextualize the work as a whole.

It would be interesting if there were a way to link it to the series as a special or extra episode, without causing duplication of entries, just to reflect the narrative continuity.

As indicated above, this should not be done.

Just to clarify: I am not proposing to include The End of Evangelion as a whole, but rather to consider the segment "One More Final: I Need You", which is formally presented as Episode twenty-six.

This episode is treated by many fans and scholars as the true narrative conclusion of the series, being complementary (or alternative) to episode 26 of the TV series. It has the title, structure and function of an episode — and not just the final scene of a feature film.

This is not to list the film again, but to recognize that this specific segment has an episodic character and can be treated as a special or extra narrative of the series.

It has originally been released as part of the movie. Consequently, it should only be listed this way.