Thank you very much for the answer. That's bad news, I was considering buying a standalone analog synth for this song but seeing that the FI4 had MIDI in I guessed I could upgrade the Akai DI to the FI and use it with an external keyboard.
If it is not possible to trigger the synth at the same time with the audio input and the MIDI in, at least it would be very interesting having the option to switch between both modes easily.
I hope you'll be succesful in achieving that. Thank you very much for your efforts!