Yes, the Tetrix motor controller supports encoders.
Remember though, that using an I2C motor controller on one of the bit-banged I2C buses will likely prove to have more of a lag than using the built-in NXT motor ports. Normally this isn't really an issue, but it could be for a segway.
I assume you want to use something other than the NXT motor ports because you want more power than they can provide, right? If cost is an issue, you could build a motor controller using e.g. a PICAXE
, and run the encoder feedback into one of the NXT motor ports for processing. This would probably also be slightly faster than using the Tetrix controller.
I'm all for gun control... that's why I use both hands when shooting