I asked them. According to the representative of Kii no bass module will be released for the Seven. There is no standard attachment slot either, the bottom is flat.
According to Kii they don’t want the Seven to become too similar to the Three. By releasing a bass module this will happen.
For people that are interested: I also asked if they release other colours besides white and grey. For now they won’t, if the Seven sells well they will.
My listening experience: for such a small speaker the Seven is quiet something. It sounds really clean (though the three sounded more neutral to me, but also more fatiguing, with very sharp treble -> could be the eq setting they used; in the setting of the listening room the Seven sounded somewhat warmer but also more likeable for me).
The bass is also very clean. I couldn’t compare the bass directly to that of the Kii Three because they only wanted to play the Three with the BXT module. Which was amazing to hear, but not something for a living room setup.