Electret microphone and FET pre-amp
You might choose to use a JFET front end for a guitar amplifier in order to preserve the higher frequency artefacts from significant attenuation when running into an amplifier with impedance somewhat less than 1 Mohm but, for an electret microphone I see no point (given that it might be powered by a 1 kohm to 10 kohm resistor).

so it's pointless connect a FET preamp out of the microphone package ?

I believe that to be a general truth but, I'm hesitant to be definite without a schematic.