While it might be possible to mod this, the secret tech of a commander is often plot-related in campaign mode, and may change for the second mission, either automatically or depending on your decisions during the first one.

For example, the Vanguard commander goes VoidTech -> Promethean (and being Promethean *is* story relevant), while the Kir'ko commander has the option of switching from Psynumbra to VoidTech if they complete a specific quest.

btw, if you decide to play kirko in the final campaign mission, voidtech is quite useful. echo domain/echo form are really nice to deal with the constant attacks from marauding stacks of autonom and paragon.