I hope you are already maintaining routing data? In normal condition when skilled labor is working then I hope you are maintaining 1 base quantity will take 5 days etc.
If yes then this is what you can try: create another routing for this material for this phase "training" and put 10 days for 1 base quantity (being 50% efficiency).
Then create a production version with validity date as 1~15 for training and 16~n for skilled routing. This way it will consider planned efficiency while planning though actual can still differ as skill improves.
Let me know if you are handling currently in some other way.
Regards
Rahul