It can be used for storage and USB-LTE devices so theoretically a LAN adapter might also be possible (beware, no Gb speeds on Hex).
Provided the drivers are available…
This page (supported peripherals) shows supported drivers and I do see some ethernet stuff there.
https://help.mikrotik.com/docs/display/ROS/Peripherals
Here is recent thread where this was also discussed
http://forum.mikrotik.com/t/routeros-usb-ethernet-support/167781/1