An external antenna is always possible providing the manufacturer supplies a connection for it but it can't just be a piece of wire.
With NFC they're not really antennas, they're inductance loops. So they really have to be tuned to a certain size/shape in order to work properly and for best performance they should be closely matched in size/shape/tuning to the device to be read.
Reading is achieved by inducing an RF field which resonates with the NFC tag when it enters the field and provides power and communication between the two devices that way.
Without this the device doesn't power on and you don't achieve a read.