can i turn non turbo'd supra into a twin turbo?

No. The non turbo engine has higher compression. You'd need most of a turbo car's engine internals, complete inlet & exhaust system and engine management unit at least.

Two turbos, at around 400 pounds each. About 2 to 300 on exhaust downpipes and fabricated bends/adaptors. Removal of your engine, stripping it, machining down the pistons to lower the Static Compression-Ratio, and rebuilding engine (hone cylinders and fit fresh rings and re-furb cylinder-head) perhaps 600 to 800, larger radiator electric fan perhaps 300, oil-cooler&take-off, 150 intercooler, new air-filter and all pipework maybe 300, about the same for an ECU re-program ("mapping") and a Water/Methanol Injection system, around 300 pounds again - basically a LOT of expense, and consider that Toyota did all the development already!

The Supra of that year was a BIG, soft, heavy and wallowing beast. IF I wanted one as a track-day car, I would simply find a turbo example, add Water/Methanol Injection and a bigger intercooler, raise the boost on an adjustable valve, and strip a LOT of weight out of the car. I bet it can shed at least 150kg, which will make it a lot faster and better-handling.

There are NO short-cuts in forced-induction!

Yeah if you chuck a good few thousand quid at it. Would have been much cheaper to buy the car you wanted originally though.

Would actually be cheaper to scrap the one you've got and buy a turbo.

You'll need many more parts than just the Turbo. Your intake manifold, headers, down pipe, intercooler, new airfilter... Not worth the money