You can't. Zigbee2MQTT is another Zigbee coordinator (like Hubitat). The coordinator runs the network, and you can't join two together. The two can run separately at the same time, or you can pick one or the other as your Zigbee coordinator if you don't want two Zigbee networks. (I have multiple Zigbee networks in my home, mostly just my Hubitat hub and my Hue Bridge network. I once had a few things on Z2M as well, though not via HASS.)
If you do want two but still want the HAOS Zigbee2MQTT devices in Hubitat, there are community options, including this. Note that this is just essentially a way to send device information (states and commands) back and forth -- nothing to do with Zigbee per se.
Or, just to make sure: do you want to just use the Sonoff dongle as a repeater? If so, you'd flash it with "router" (repeater) firmware instead of "coordinator" (hub-ish) firmware, then you can join it to Hubitat like any device. The device can run only one or the other at a time. This would not involve Z2M at all (unless you have another coordinator there and want to join it as a router/repeater to that network instead).
