VTOL Motors enable in FW mode

I would like to fly at -20 °C, however I use seperate battery for VTOL motors.
The chemistry in the battery needs approx 10 seconds to wake up from -20°C. After that, full throttle can be used and VTOL can land.

Solution would be to put VTOL motors to idle few seconds before transition.
How can I do that ?

Thanks, Sam