My trans is slipping from 2nd on. I tried the fill up procedure with right trans temp twice. To rule out low trans fluid as a cause, I would like to 'overfill'.
We have a nice cold day today, so I could get the engine running, shift through gears (10 sec each), raise car, get fill plug out, insert the fill hose, and pump ATF in until if overflows. All with a much lower (say 10-20 deg C rather than 35-45 deg C as recommended). I assume this would cause a higher fill level.

What if any are the risks involved in do so?
If it solves my problem, I would know that it has to do with the ATF level (or pickup tube or filter or else).
If it does NOT impact the problem, I would just get the car to 35 deg C and take the fill plug out and let the extra fluid drain.

Input, please!