There is no way I am aware of to connect an Arduino to DeviceNet, or any of the major industrial Fieldbus systems.
Since the robot's I/O update is, at best, every 12ms, Panic is correct -- directly bit-banging the coils of the stepper driver may not be sufficiently fast.
What you could, perhaps, do is this:
1. Purchase an appropriate number of DeviceNet slave I/O modules using dry-contact (relay-style) inputs and outputs
2. Connect the Arduino's unused I/O pins to the DeviceNet I/O
3. Write Arduino firmware to handle the low-level extruder speed control in real time
4. Have Arduino firmware accept speed/direction command updates from the robot every 12ms, and "fill in" the period in between on its own
5. Write robot background task that updates DeviceNet I/O speed and direction to the Arduino every 12ms, based on "G-Code" commands and actual realtime TCP speed.