@Taka , as you are aware that this configuration is not supported, so you would be testing it at your own risk (in terms of drone flying normally after all the changes and potential issues with motor overheating). Electrically, Starling 2 is compatible with 3S. The entry point of power into Starling 2 is the mini ESC, which is rated for up to 4S Lipo.
https://docs.modalai.com/voxl-mini-esc-datasheet/
Now, regarding the motor.. you would need to do a ESC motor calibration using the following procedure : https://gitlab.com/voxl-public/voxl-sdk/utilities/voxl-esc/-/blob/master/voxl-esc-tools/calibration.md?ref_type=heads (mainly update the battery voltage, test and update rpm limits, calibrate the feed forward (pwm_vs_rpm) curve, probably lower (a little bit) start-up power, confirm start up is smooth). You may also want to slightly lower the kp term (proportional term in the rpm controller, to account for increase in battery voltage, so that the controller does not become more aggressive). Maybe lower that from 100 to 66-75 or so.
These should be the ESC params that are loaded on your ESC : https://gitlab.com/voxl-public/voxl-sdk/utilities/voxl-esc/-/blob/master/voxl-esc-params/D0014_starling_1504_3000kv_ms_props/D0014_starling_1504_3000kv_ms_props.xml?ref_type=heads . You can double check this using voxl-esc-verify-params.py script and then modify these params with your changes. The esc tools are already installed on voxl2 in /usr/share/modalai/voxl-esc-tools.
The main issue you may see is if you apply full power to the motor continuously, it may overheat due to the fact that it was designed for 2S. However, you can artificially cap your maximum rpm (in esc params) to prevent the motor from going to higher rpms (and update your px4 params for max rpm accordingly).
Depending on what you are trying to achieve, you will probably have to put some SW protection and prevent the motor from overheating.
If you are OK with waiting for a few days, I can give the motor a spin at 3S and just confirm nothing BAD will happen.
Alex