If I buy a Wi-Fi Ipad can I add the data package later

NOt a wifi only. Now wifi/cellular, yes you can add the data later. Just make sure the device you get is set for the network you plan to use.

Now if it's wifi only, you can still add data, but not directly to the iPad. What you do is use a device like the Mifi, which is a battery operated wifi hotspot that uses cellular that you can then connect your iPad do. (which is honestly what i'd recommend. Yes, it is more clumsy to have two devices, but you have maximum flexibility for networks and your iPad isn't locked to any one provider.

In addition to Skydiver's response, if you have an iPhone or smartphone that permits setting up a personal hotspot, you can use that to provide data coverage to the iPad. Personal hotspot is really just a very limited range WiFi created by the cell phone that the iPad can access and use just like any other WiFi. The only caveat is that the cellular service provider has to allow use of personal hotspots, and some charge for that provision.