I'm sure it could be added. I can look but not sure how much effort I will be putting toward OMV 5.x plugins with OMV 6.x on the horizon. You can always mount 9p from the command line and then use the sharerootfs plugin to make shared folders.
As mentionned in all this thread, I have this 9p mounted through fstab. From my point of view, it works perfectly from OMV in SSH.
Using sharerootfs is not working for me (see the issues I have in OMV GUI and attached services : SMB in my case).
Using mergerfs makes it work...