You have to make the cable yourself when the unit is new so there is no problem replacing it. The cable is made up of 2x5 Dil IDC sockets which can be bought from an electrical component supplier, and the appropriate cable. How to make the cable is in the instructions; you just need to make sure you get the sockets the right way round.

I'm not sure if lengthening it would cause any problems but my guess is that it would be fine. If the unit is second hand there is a good chance the one you have has been cut down anyway.