You can put the device on the same subnet that the cRIO lives on and communicate to it with TCP palette functions. If it is TCP modbus device there is library alread for it you can use. I don't know how your work area is setup, but my HMIs conenct to the cRIO we run throught the facility routers/ office ethernet. The routers are in a closet somewhere. This is fine for the user interface connection but I would not have the cRIO go through the office routers to control a device that helps run your process/ machine.
If you want to create a private subnet so that nothing other than the cRIO can communicate to the device, and keep all hardware local to the cRIO, you can configure the cRIO's second ethernet port ETH1 as ethernet, and connect the device and the cRIO to an industrial type router mounted adjacent to the device and the cRIO. Then the only the CRIO acts as buffer between the device and the outside world.
