It is not possible to change or modify the internal hardware components of an iPad - such as adding cellular support, or increasing internal storage or RAM. What you have is fixed for the life of your device.
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. For iPhone:
How to set up a Personal Hotspot on your iPhone or iPad – Apple Support
If you don't have a Cellular phone that can provide a WiFi hotspot for other devices (such as your iPad), a cost effective alternative is to invest in Mobile Broadband Router. These are offered by Cellular Carriers by different names - "MiFi" or simply "Mobile Broadband" being the more common. The Broadband Router will simply require addition of a Cellular Data Plan adequate for your needs.