Saad is correct. The EX60 does not have support for an external microphone. It does support headsets with dual 3.5mm plugs, and with TE6.0 we will be adding Bluetooth headset support.
The EX90 has the same capabilities - headsets with dual 3.5mm plugs, and Bluetooth support in TE6.0 (which is due before the end of the year). However, the EX90 does also support the use of an external mic - as Saad says, it is the Performance Mic 20 with part number CTS-QSC20-MIC=. The EX90 will automatically recognize this when it is plugged into the 3.5mm jack that is normallly used for the 3.5mm headset.
Assuming you are talking about the EX90, when you plug in the external mic CTS-QSC20-MIC=, it automatically overrides the internal microphone. For simplicity, you cannot use the internal microphone at the same time as the external mic. It is one or the other - plug in CTS-QSC20-MIC= and you are now using it. Unplug it, and you are using the internal mic.