I think it’s viable in only one situation and in 10 easy steps.
Step 1: Pull up the parts catalog and identify all the different parts. (Is, HVAC, seats, wiring harness, etc.
Step 2: Find all the parts via eBay and purchase them. It’s roughly $800 for the seats, $500 for the C panels with 3rd row vents.
Step 3: Find a Tesla authorized body shop.
Step 4: Convince said body shop to purchase all the parts that can’t be sourced via eBay. Seatbelts, wiring harness, etc.
Step 5: Install all hardware parts.
Step 6: Find an active Tesla tech, or independent Tesla tech with Toolbox access.
Step 7: Convince Tesla tech or independent tech to log in and reconfigure the car to be a 7 seater so software recognizes the new hardware.
Step 8: Crash test it.
Step 9: Purchase another 5 seater MX and repeat steps 1-8 until you’re comfortable and convinced it is safe.
Step 10: Once satisfied, purchase another 5 seater and perform steps 1-7 for your final 7 seater conversion.
Piece of cake.