Since the car was designed to accept the third row safely then yes, you can add it afterward.
I’m betting that the mounting points you refer to have a threaded fitting that’s installed should you choose the third row as an option at the time of purchase. Ask the parts guy at the dealer to look these fasteners up for you and show you the “exploded view” drawing of their proper installation.
You may also want to see if there’s a plug for a “seatbelt disengaged” warning light that you should be connecting. If so, there should be a plug dangling from the seat bottom for the “occupant” sensor part of the warning system. The warning lights system (if you have one for the third seat) senses the presence of a person as well as whether the seatbelt is engaged.