Can you still activate cellular after when you bought an iPad without that feature?

If you bought an iPad without the cellular hardware, the hardware cannot be added later.

If you bought an iPad with the cellular hardware but did not activate it with a carrier, you can activate it later.

If your iPad does not have a SIM slot, then you have a WiFi Only model of iPad. WiFi Only models do not have any Cellular capabilities - and hardware cannot be retrofitted. It is not possible to change or modify the internal hardware components of an iPad.

If you do have a WiFi+Cellular model iPad, you can add a DataSIM for your chosen Cellular Carrier - or for recent models of iPad, activate the eSIM:

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

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 perhaps also need to be aware that WiFi Only models of iPad do not have an GNSS/GPS positioning capabilities - which will impact location services on the iPad. If you have need for precise location, or moving-map Apps, you would be well advised to consider a WiFi+Cellular model iPad from the outset. Choosing a WiFi+Cellular model does not commit you to provisioning cellular service with a Carrier, but does provide flexibility should your needs change.

Unfortunately no, you can't add cellular capabilities to an iPad if it didn't come with it from the factory. The hardware necessary for cellular data connectivity (like the cellular modem) isn't present in WiFi-only models of the iPad. If you think you might need cellular data in the future, it's usually a good idea to purchase a cellular model up front.