Q: Can an IPAD 2 WIFI be upgraded to WIFI Celluar?

No.  There is no way to add the cellular hardware to a Wifi only iPad. 

You could however get a mobile hotspot or MiFi from your Cell carrier and use that to provide internet to your iPad when no Wifi networks are available.

https://www.google.com/search?q=Mifi&oq=Mifi&aqs=chrome..69i57j69i60l4j0.827j0j1 &sourceid=chrome&ie=UTF-8

If you have an iPhone,

iOS: Understanding Personal Hotspot

http://support.apple.com/kb/HT4517

Use Bluetooth to tether your iPhone, iPod touch, or iPad

http://www.macworld.com/article/1159258/bluetooth_tethering.html

How to Connect an iPad to an iPhone Via Bluetooth Tethering

http://techtips.salon.com/connect-ipad-iphone-via-bluetooth-tethering-25472.html

 Cheers, Tom