FET impedance converter (for a non FET electret mic)?
Let's start with the electret microphone basics.
The actual microphone is formed by a (very very) thin sheet of mylar film that is charged. This is connected to the metal body of the electret capsule but insulated from the pick-up plate. The mylar film and the pick-up plate form a capacitor. The charge on the mylar film produces a voltage difference between it and the pick-up plate. (V = Q/C)
As the mylar film is moved by the sound waves it changes the distance between it and the pick-up plate and changes the capacitance value as the distance between the 'plates' increases and decreases.
This is effectively what your electret microphones consist of.
Most electrets contain a FET which converts the very high impedance of the 'capacitor' voltage into a more useful form. It consists of a FET with the gate connected to the pick-up plate, the drain connection as a '+ pin' and the source connected to the ground (metal body).

You could add a FET  but given that electrets are so inexpensive, by the time and money you would spend adding a FET to your electrets it would be a lot easier and cheaper and more reliable just to buy a new electret microphone with the FET built into it.