Yes,
external functions are currently not natively supported in CADET-Process. But since CADET-Process ultimately translates its config to the CADET-Core interface, you can modify it before running the simulation.
So you could to something like the following:
from CADETProcess.simulator import Cadet
simulator = Cadet()
cadet_config = simulator.get_process_config(process)
# Specify your external function here
from cadet import Cadet as CadetAPI
cadet = CadetAPI()
cadet.root = cadet_config
cadet.run_simulation()