The first question I have is if you really need mesh. That is, do you need routers to extend the range of the network? If you do not, then the 802.15.4 protocol would be better suited for your application.
If you need routers to extend the range, then Digi Mesh would be a better solution.
Each XBee is based off of a Different Chip set/processor. For that reason, you can't convert a Zigbee Enabled product such as the XBee ZB module you have to an 802.15.4 or Digi Mesh product. You must purchase the correct module instead.