As mentioned you will not be able to join them to an existing Zigbee network unless you manage to hack them and flash them with unofficial Zigbee Router firmware image.
It will be so much easier to just buy a few “IKEA Trådfri Signal Repeater” (which is a purpose built dedicated Zigbee Router device) or buy a few more Sonoff Zigbee USB dongle adapters and reflash those with Zigbee Router firmware to use in USB changers for the best possible performance.