If you want to avoid the $10/month fee to add it as a device on your Share Everything plan, you can swap the micro sim from your phone in and use the data.
I have an old unlimited data plan and swap the sim between my phone and the iPad all the time. If its going to be in the iPad for a while I forward my phone number to a google voice line that actually rings on the iPad so I dont miss calls.
We did get an iPhone5 for my wife and I confirmed that using a nano-sim to micro sim adapter, it still works.