Thjs is hard to convert to a clause because of "has become". Do you think it should be "After he has been trained" or "After he was trained" or "After he had been trained"?
ex)Having been trained for 2 years, he has become very skillful in the trade.
Or After training
PS If it's hard to convert to a clause, might that not be an argument for not trying to do so? You could easily use something like He was trained for two years and has..., though.