If I understand your question correctly, you want to access the internet from Zigbee devices?
If you have a regular wireless access point available nearby, I would use the USB peripheral on your CC2538 and connect a wifi module to it. You would then need to run some translation code to convert data from the Zigbee protocol to the regular Wifi protocol.
Without a conversion, it is not possible as far as I'm aware since the Zigbee protocol is implemented on the 802.15.4 standard and normal WiFi operates on 802.11. You could also look into a serial to Ethernet conversion, but would again need to implement custom code to convert signals.