Thanks.
Until today it has been possible to merge only units that were less than 5 * in rarity and whose potential was increased. The original 5 * rarity units are not possible for me to merge. I imagined that this option would be unlocked after making some progress, but considering its response to fusing units of original 5 * rarity it was supposed to be available from the beginning of the game.
Sorry if the text was not so clear. I only have notions of English, so I used google translate to write this text.
Edit: Something similar happens when inheriting skills. I can only inherit from the 5 * units that were previously of lesser rarity
