Rulings for modifying joysticks in 2007?

[Japper]

I did not find this topic in any of the posts or the 2007 rule book and I am wondering...

Per 2007 rules, can we modify joysticks or controllers to use
in place of the kit provided joysticks?

I am considering modifying a couple PS1 controllers but would like to know if anyone has found anything in the 2007 rules which dis-allow this sort of thing.

I am only interested in hardwiring the controls to the DB15 connector(s)
and not messing with (by-passing) the serial circuitry inside the controller.

Please advise (and thank you for your advice.)