Non-cellular (WiFi Only) models of iPad have no cellular hardware components at all; no SIM, no eSIM, no cellular antenna…
WiFi Only devices are entirely reliant upon WiFi, Bluetooth and (if connected) a wired network connection for connectivity.
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
You may also wish to consider that only WiFi+Cellular models of iPad have GNSS/GPS capabilities - this being a function of the iPad Cellular chipset. WiFi Only models of iPad have no GPS capabilities with which iPad Location Services can derive location. This fundamental difference may significantly impact your ability to use mapping Apps - in particular if real-time positioning (e.g., a moving map) is required. When using a mobile phone’s hotspot, the phone does not share GPS information derived from its own positioning capabilities.
WiFi Only iPad models can only “infer” their location from a database lookup of neighbouring WiFi networks for which the geographic location is known, or the geographic location of your public IP address. For static applications in urban locations, where the location of IP public IP addresses and WiFi networks are known, an “inferred” location can be relatively accurate. However, in rural locations, or when using a Cellular/Satellite/VPN connection, location accuracy of an inferred location [if it works at all] may be very inaccurate.
All this has significant implications for non-static mapping applications. If you need to have moving maps, or have future flexibility to do so, you may need to consider a WiFi+Cellular model of iPad. Choosing a cellular model does not commit you to adding a cellular service plan for the iPad, but ensures that you have immediate access to GPS/GNSS positioning services.