IMHO:
If it is possible? Without causing discontent with your mechanic. Yes. Why - logically it's sound process and should be followed when it was in place for you when the Manufacturer made it possible for the vehicle to be maintained.
Keywords:
Made It Possible.
Nowadays? It is not possible unless you poke holes into the container and drain it yourself. A Mechanic once did this using oil cans that were packaged and sealed and they put it's contents into it's proper place in the car for you at the proper amounts when you came into the local gas station - They called it Full Service.

If your mechanic can't figure this out or says "they" don't recommend it, then there is little I (or anyone else following recommended procedures; for what they see in manuals they can review themselves) can offer you. But I do know how to do it using the very same things they keep on the transmission, every transmission - because they still have to fill it let alone empty it when they need to service it.
And No, you might not need to poke holes into the transmission to do this - but it will take photos - good, detailed photos of your transmission and all its' case - so I can show you where to drain, and where to fill.
If you own an Automatic, it's a little more tricky than a standard Manual Transmission - but the process is possible.
Re-discuss this with your mechanic then make a decision on which route to take before this goes down a rabbit hole that many will not feel comfortable with - for you may have to do this alone using tools and processes that can put your car in the service shop anyways.
This requires you to have some knowledge and inclination to become the responsible owner and mechanic of your own vehicle - that may not sit well with many that have come to accept the advice of the mechanic they once trusted their car to.
