Adding a wireless account

Do you have a WiFi Only or WiFi+Cellular model of iPad? If you have a WiFi+Cellular model, you can easily add cellular service:

Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

While you cannot add Cellular capability to a WiFi Only model of iPad, you can access Cellular service using a WiFi hotspot. There are two main ways to provide a WiFi hotspot:

1) A mobile phone - that can provide a WiFi Hotspot. An iPhone is ideal, as the hotspot connection can be initiated directly from the iPad. Other brands of mobile phone will usually suffice, but the hotspot connection must be manually enabled each time that you wish to use it. It follows that the Cellular contract must allow connection via the WiFi hotspot (otherwise known as tethering).

How to set up a Personal Hotspot on your iPhone or iPad - Apple Support

2) A flexible alternative is to purchase a mobile broadband router - often known as MiFi - and an appropriate cellular airtime contract from your chosen Cellular provider.

A hotspot connection can also be used with WiFi+Cellular models of iPad. In this regard, both WiFi Only and WiFi+Cellular models are equal.

I hope this information, in exploring your available options, provides some helpful food for thought.

there are wifi ipads and there are ipads with wifi and cellular

of you have one of the former (the later cost more money) then you can't add a cellular plan to it, but you can have it share the cellular plan of an iphone you own