Source: https://www.yamahasynth.com/ask-a-question/effects-for-external-inputs

The new CP88/CP73 (2019) have an external input to handle a LINE level mono or stereo input. The concept is consistent with it being the bottom keyboard in a stack or working in a multi-key rig — perhaps you have a YC61 or a synth on the upper tier. You can connect the other synth/keyboard through the CP... the signal simply passes through unchanged, unaffected... neat, convenient;
__You can freely use the front panel of the CP for the CP. And although the new CP can route its own audio directly to your DAW via USB, this does not include the external audio input.
__The USB “To Host” port can be used as an external playback portal (iOS compatible).
Design concept is real-time access... quick, agile, flexible. Less menu diving, less setup ahead of time.

If the microphone provides a line-level ("compatible" ) output then yes.

A lions-share of dynamic microphones do not require power and therefore would be "compatible" with these inputs even though the intended usage of the audio inputs is more to connect another keyboard's output rather than microphone(s).

The above quote also details what you get from the audio inputs. That the external audio does not route to USB seems to contradict with your phrase: "has a built-in audio interface with a gain control" - because "audio interface" implies conversion of analog audio to USB and "gain control" implies the two audio inputs discussed in the above quote.

Thank you for the detailed and quick answer ! 🙂