Serial ports are implemented on the HAL level, for Marlin 2.0.
So if your particular MCU type has not yet has methods for dealing with Serial_2 options implemented, then there are no (current) straight forward option of how to get Serial_2 options to work for your MCU.
You might however be lucky to find someone who has done an eksperimental implementation, or a search on github/marlin about serial_2 might give you some clues of how to do your own eksperimental implementation.
So if your particular MCU type has not yet has methods for dealing with Serial_2 options implemented, then there are no (current) straight forward option of how to get Serial_2 options to work for your MCU.
You might however be lucky to find someone who has done an eksperimental implementation, or a search on github/marlin about serial_2 might give you some clues of how to do your own eksperimental implementation.