We already support the use of two joystick simultaneously, one for translation and one for rotation. There are also
keyboard shortcuts for certain systems like the sextant and DSKY.
Putting those things on a 'button box' shouldn't be an issue. bru57000 already did this with an external DSKY as seen in his video:
EDIT: Actually looking at this video again I think he implemented his own interface with NASSP over TCP instead of keyboard commands, you'd need it for the display.
Building a fully fledged simpit would be quite an interesting thing to see, it would take quit a bit of work to build such a thing. NASSP currently does not support it as controls not exported via keyboard shortcuts can not be manipulated. Of course if the need arises that can be implemented.
So yeah, theoretically it would be possible. Someone just needs to be crazy enough to do it.