Installing WWAN Card.

1. Are you certain that your system has the WWAN antennas? It's common that if a system were ordered without WWAN option it would not include WWAN antennas. You would of course have WIFI antennas.

2. If you do indeed have the WWAN antennas, then yes, you could install a WWAN card and attach the antennas. Be careful when disconnecting and connecting the tiny antenna plugs that fit over the tiny posts on the card (kind of like snap fasteners on a coat). They are fragile. 

3. You would need to choose a card that is compatible with your desired provider's service.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard. (Repeatedly touching a metal case part is not sufficient mitigation with modern components.)