I would like to install a sim slot on my ipad. Whom do i approach

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

A WiFi Only iPad cannot be transformed into a WiFi+Cellular model. You can, however, 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

Really...

This is never going to happen.

A cellular data service iPad is more than just a SIM card slot!

A cellular data service iPad has the addition of an internal MODEM chip, internal cellular and GPS antennas, a modified version of iOS/iPadOS, that includes all sorts of cellular settings and info in the iOS/iPadOS Settings app.

Sell or trade-in your WiFi ONLY iPad and purchase a cellular data service iPad model, instead.

Good Luck!

No way to do it, sorry.

Buying an iPad, you have two choices: Wifi Only, or Wifi + Cellular. You can't "convert" an Wifi Only device to host a SIM data card.

Regards

Giulio