Section 8, R78, bullet point A, at the bottom of page 91 of the Complete Game and Season Manual, as of today (1-15-2017):
Additional switches, sensor modules, CUSTOM CIRCUITS, third-party modules, etc. may also be placed on the CAN-bus.
You can add whatever custom devices you want as long as the roboRIO, PDP, PCM, etc are connected to the CAN bus legally and the custom device(s) do not interfere with, block, or override other signals (See rules R61, R75 to R78).
I'm curious to know what custom devices you intend to build, if you don't mind sharing