Pairing Echo Plus to HE?

Is it possible to pair the Echo Plus to Hubitat. They are both hubs, but if possible I would like to create one strong Zigbee mesh rather than two competing meshes.

Particularly since the new Eero 6 has a Zigbee radio that integrates with Alexa/Echos - it would be good to use it as a repeater.

No, it isn't. They're both zigbee coordinators, and a zigbee network can have just a single coordinator. I'm not exactly sure how the Eero 6 / Echo Plus integration works - perhaps the Eero 6 appears as a zigbee device to the Echo Plus?