Hans has answered, not discouraging me at all.
Connecting more I2C modules is possible, and there is an easy place to connect them, the OLED display.
He also has shown me
This video. The device is the same FM chip, but still better, it has an amplifier incorporated. It could give sound externally, but not through the SF3 to the internal EP mixer.
I think it is a feasible option but.... needs some time to implement and test the software to control it, as it is not only like to copy the available SE-ONE or RSF3 code to the SF3.
Now all Hans' time is for the RSF3, but maybe on a future....