First create a new sink:
pactl load-module module-null-sink sink_name=zoom_input sink_properties=device.description=zoom_input
then remap into a Zoom monitor source that Zoom will allow selection of:
pactl load-module module-remap-source master=zoom_input.monitor source_name=zoom_mic source_properties=device.description="zoom_mic"
Then create as many new virtual sources as microphones you need to merge together with
pactl load-module module-loopback and finally go into Pavucontrol, select Virtual streams, and assign different physical sources in the dropdowns. Then select zoom_input in Zoom as source.
Good reference: Play audio output as input to Zoom