We asked the same question to KUKA some years ago, axis 6 on the RDC is not possible to be de-coupled. Every time we order a new robot the 2 additional axis are put on channel 6 and 7 of the RDC from the factory, and we need to open the RDC box and move them to channel 7 and 8.
Before we used the kuka external axis, we used another brand of servo motor with an interface PLC between the robot and the servo controller/axis. As the axis are async with the robot this was not an issue. That could be a solution, as decoupling them does not involve the KUKA anymore.
This was all about KRC4, to be honest I worked today a similair robot but with KRC5 controller, and I did not have to open the RDC box to move something. So maybe something has changed in KRC5.