You could probably find the solenoid valve that controls the tool change air blast and tap into the line downstream of it with a solenoid valve controlled from a user M function.
But this might not be a good idea, even turning on the normal air purge might not be a good idea, because you do not know is the seal is designed to be pressurized while the spindle is running. I have had machines that fed air into the spindle when it was stationary and the seals were designed to retract when the air was off; they only extended and formed a seal when air was present under pressure. One time I accidently turned the air on with the spindle turning and the seals acted as a very efficient brake; and never did seal correctly after that.
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