I'm not sure what you mean by StdService. That's the abstract base class all the services inherit from.
In any case, if a service is to augment a record, it must check what unit system the incoming record is in, then convert what it is about to add to that unit system. There are many utilities in weewx.units for doing this. In particular, look at weewx.units.convertStd(). Also, look at ValueTuples, also to be found in weewx.units.
Say you are about to add a wind speed that is m/s. It would look something like this (NOT TESTED):
def new_archive_record(self, event):
my_wind_speed = (obtained from somewhere)
# Create a Value Tuple:
wind_speed_vt = weewx.units.ValueTuple(my_wind_speed, 'meter_per_second', 'group_speed')
# Convert to the same unit system as the record
wind_speed_converted_vt = weewx.units.convertStd(wind_speed_vt, event.record['usUnits'})
# Put the converted value into the record:
event.record['windSpeedNetatmo'] = wind_speed_converted_vt.value