What do you mean by cards? I'm assuming you mean the three cRIO modules which are NI 9201, 9402, and 9472 (kind of like
this), and not the analog breakout/solenoid breakout that sit on top of the modules.
Regardless of if it's legal, you probably won't be able to do it. The connection between the cRIO and the modules is not a normal db-15 connector commonly used for serial ports. Even if you did find the right connector, there would be a lot of undesirable interference. The module was designed to be plugged straight into the cRIO with no extension cable. They made the connection as short as possible by mounting the socket directly to the cRIO's PCB to avoid interference.