I am buidling a delta printer and stumbled over the "linearDeltaPlusZ" topic, which is exactly what was looking for to get a tool axis which could be on the same level as the effector at all times.
M669 K9 will tell the printer that I got a additional linear z Axis, but I cant find the allocation parameter for the stepper driver. ( I won't use a stepper driver for the extruder so E0 and E1 would be available)
I suggest you look at https://duet3d.dozuki.com/Wiki/ConfiguringRepRapFirmwareDeltaPrinter#Section_Adding_additional_towers_to_carry_flying_extruders. Pure Z movement to match the effector height isn't supported, but you can approximate it if you want to by specifying a very long "rod length" for the 4th tower.