Is it possible to add cellular to my iPad?

Presumably you refer to an iPad - and not an iPod.

It is not possible to change or modify the internal hardware components of an iPad. 

Whilst a WiFi Only iPad cannot be transformed into a WiFi+Cellular model, you can use the hotspot feature of a mobile phone - or a Cellular Broadband Router (often known as a MiFi Router) - to access Cellular networks.

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

If you have purchased directly from Apple, in most countries/regions, you have up to 14-days within which you can unconditionally return your iPad for exchange or refund. If this applies to you, you’ll be able to return and exchange the WiFi Only model for WiFi+Cellular - obviously paying the difference.

If instead you purchased from another retailer, you’ll need to verify the retailer’s returns policy.

Otherwise, you can still use a MiFi or mobile phone hotspot - as outlined above.

No. If desired, check the store's return policy.

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