A drive that is formatted ext4 can be integrate without any problems. You just mount the filesystem normally in the File Systems section. Then you can create new shared folders with OMV and move data into them. Or add existing folders to the shared folders. If you are adding existing folders you need to select the path. Also, be aware that the chown and chmod will be altered. You may not want this for an existing folder if you plan on moving it back to another system.
Then perhaps there is something wrong with my OMV install?
Before installing usbmount, if I plug a thumbdrive into a USB port on my OMV box, I have to log into the GUI, navigate to Storage | File Systems, click on the newly discovered device, and then click on Mount. Then I can access the thumbdrive.
By saying you don't think having usbmount installed was necessary, are you implying that stock OMV should automount devices plugged into USB ports?