How do I place the seat back 20 cm with a full suspension bike?

The only safe answer is to buy a bigger bike frame.

When your adaptor fails, you will descend rapidly onto the rear wheel. The remaining original seatpost has an excellent chance of tearing your chest open, and impaling your head up through the jawbone.

While this is definitely creative, its absolutely not the right answer.

Buy a bigger bike frame.

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If you're under 20, then there's a good chance you're still growing. Even more reason to buy a larger frame bike.

The classic solution is banana seat and sissy bars. You'll lose the rear suspension, but the chances are that it's not going to work anyway because you'd be sitting way more back than what the suspension was designed for.

Edit: Looks like suspension sissy bars do exist. It's going to be an interesting problem to balance the frame and seat suspension.

There is a simple, pro solution to this and it is have a framebuilder make you a seatpost that can do this.

A simple iteration would be a bunch of segments of chromoly tubes, which may need the part that goes in the frame to be turned, and then have the last segment be a 22.2mm one such that it can take a cheap saddle clamp. A more sophisticated version could have guts transplanted from another seatpost.

There is an open question with all of this whether your frame can handle this kind of leverage at the seatpost opening without flaring open. My guess is probably not, and there's no way to solve that problem. There are bikes that are designed to have much more setback than others, i.e. most Electras and their many copies, so you are not playing in a new sandbox here, but those frames have slacker seattubes to keep the aforementioned leverage down.