Can I add non-Sonoff devices to my Sonoff Zigbee Bridge?
Yes, you can. As Wikipedia describes:
Zigbee is a low-cost, low-power, wireless mesh network standard targeted at battery-powered devices in wireless control and monitoring applications.
Source: Wikipedia
That means different manufacturers can produce Zigbee devices that use the same protocols to communicate with other Zigbee devices.
What does Sonoff say about Zigbee compatibility?
On their product page for the Sonoff Zigbee Bridge Sonoff shares the following image:

As you can see the following product types certified bij the Zigbee Alliance are supported:
- One gang smart switch
- Wireless door/window sensor
- Temperature & humidity sensor
- Motion sensor
- Water sensor
Other info
More product information about the Sonoff Zigbee Bridge can be found at our Device lists: Sonoff Zigbee Bridge