Add cellular data to a wifi ipad?

No, a wifi-only iPad doesn't have the necessary hardware within it. As an alternative you can use an iPhone as a personal hotspot (if your carrier supports it) or get a mifi device and use that as a mobile hotspot.

Contact your cell provider to see if they offer hotspot plans.

It depends upon your carrier and what tariff that you are on with them - it may be an extra amount in order to enable it on your contract with them. If you have a Settings > General > Cellular Data > Personal Hotspot option on your phone (personal hotspot) then you are already on an appropriate tariff with them, and the iPad should see the phone as a wifi network - depending on what amount your current data allowance is, you may want to increase it if you plan on using it a lot with the iPad.