I don't believe you can mod these to be Midi switchable directly, however, I used to have a Nomad 100 and came across a custom box made by Mesa that ported the switching functions of the nomad out to individual 1/4 jacks. These could be controlled by individual latching switches or by a midi controlled switching unit.
You might want to contact Mesa and see if it's still possible to have them make one of those. It worked very well and allowed me to better integrate my Nomad with other gear.