It's not actually that hard, if you can accept some steps in the voltage.
Buy a transformer rated at 220V in / 5V out at 50 A, and one at 15V, 50A out.
Their VA ratings are 250 and 750 VA which isn't too bad.
Now connect their outputs in series, and also in series with the load.
Use 5 solid state relays per transformer so that you can connect each primary either way round to the incoming mains, (4 switches) or short it out (5th switch)
You must make sure that the primaries are shorted when not connected to the mains.
Each transformer will then give you plus or minus its voltage, or nothing.
That will give you +20, +15, +10, +5, 0, -5, -10, -15 or -20 V
Your relays only have to handle about 3A, so you would use 600V, 10A ones to be sure, but they are not too expensive. You need a microcontroller to run it all if you want it automatic.
Usual warnings. Although this can work, if you can't see why this could be dangerous, you don't know enough to keep yourself safe.
You can test the scheme with low current transformers, and with low loads on the big transformers.