I have never tried any of this, but based on the communications protocols they use, it seems completely possible to use both NXT and EV3 sensors.
The digital NXT sensors (ie. ultrasonic) use I2C to communicate, likewise the NXT port on the digital sidecar was just connected to the I2C pins. I believe you would just need to make/modify a cable to connect the appropriate pins to the I2C port on the roboRIO and write a driver to communicate with it.
Some NXT and EV3 sensors (ie. light sensor) use analog signals to communicate. Using these should be as simple as connecting the correct pins to an analog input on the roboRIO and figuring out how to convert the voltage to a value.
The digital EV3 sensors use UART (serial) to communicate, so they can be connected to the UART port on the MXP (not the RS-232 port, at least not without a converter). Again, you would have to write a driver.
If you do any of this, the most important thing is to make sure you are supplying the right voltage to the right pins, to prevent damage to the sensors (the roboRIO is pretty well protected, so it would be fine with most mistakes).
These links might be helpful:
http://lechnology.com/2014/09/using-...-on-any-linux/
http://www.ev3dev.org/docs/sensors/
http://brweb.haltonrc.edu.on.ca/2022...ExtremeNXT.pdf (a lot of information)