Is my request absoluty stupid ? Is it already built-in with EarMaster ? Should I shut up and work harder ? :o)

Both Yes and no (kidding ;-) ) . The thing is the chord identification exercise is working on a theoretical basis. It's all about training your ear and helping it recognize qualities ...

Is there a way to use the tutorial with open chords in "chords identification" ?

A CMaj7 is rarely played with C E G B in that order, not on the guitar anyway, and I don't think on the piano neither. The closeness of the notes makes a 4-notes chords very difficult to recognize. It's certainly a ...