Hello,
Basically KUKA|prc does not know anything about the technical specifications of the axis, there is the geometric model and then the assignment to e.g. E1 as the variable.
So you can build any setup, it's just important that you can calibrate it exactly. There are strategies both for calibrating linear and rotary axis, so if the mechanics are sound I don't see a problem there.
While the KP1 is not included in KUKA|prc, I believe the guys at the AA have used one for the pavilions out of logs.
Regarding the hardware, the only requirement for a properly synchronizing axis is that it has to be a KUKA motor. The other mechanics can also come from another manufacturer (or from yourself). E.g. for linear axes, Güdel is very popular.
Note that you also need to ensure that your controller is compatible with the additional axis, you might have to retrofit it otherwise, which can be quite expensive.
A good way to start is probably to ask KUKA for an offer for the KP1, including any potential extra costs for your robot controller. If you give them the serial number, they will know how it is equipped.
Best,
Johannes