How do you know? Have I missed something in ICOM's
manuals... or have you tried these experimentally? :-)
Yes, you did. IC-7000 manual, page 3:
!3MICROPHONE CONNECTOR (p. 10)
Modular-type microphone connector—accepts the
supplied microphone (HM-151).
•The optional OPC-589 can be used to connect an 8-pin
microphone such as the SM-20, if desired.
•A microphone connector is also available on the rear
panel. DO NOT connect 2 microphones simultaneously.
This kind of prohibition, you ignore at your own peril.