How can I, theoretically at least, use switches connected to arduino board and then use arduino to "convert" those signals and send them to the computer?
That one is simple and there are lots of examples around, this is what I have done:-
http://www.thebox.myzen.co.uk/Hardware/MIDI_Footsteps.html
How does Ableton/Guitar Rig/others interpretate Midi signals
I think you need to look at the Ableton manual for that.