|You don't need an STC to have something that is already incorporated in the type design, but you need to incorporate everything that is in the type design when installing it, and it might not be very easy to find that data. The first person who says 'just use the IPC' gets 50 lashes with a wet noodle!
Due to the huge amount of modification required, probably the only practical way to do it would be to find a set of wings with LR tanks installed and perform the necessary maintenance required to install them (installing control cables, controls, flap motor, testing fuel tanks, vents, and sending units, wiring if it has been damaged or removed, probably a zillion other small details). Still not an easy undertaking.
I would imagine a ship set of good wings with LR tanks commands a premium over standard tanks.
This is probably why there are lots of STCs for LR tanks out there, like Flint Tips for the 172.