Is it possible to use my (3d printers 12V 20A power supply) in series with 12V unknown amperage pc power supply to gain at least 24V 20A for my printers new hotend?
The PC power supply (-) lead will be connected to the Earth terminal of the AC Mains plug. The other supply may be isolated from the Earth terminal - or may not.
If the (-) terminal of the other supply is in fact connected to the Earth terminal, it's usually pretty easy to open the case and separate the (-) terminal from the Earth terminal.
Then just connect the two supplies in series.
I'd be tempted to install large diodes in anti-parallel with each of the outputs of the power supplies. That is: Cathode to (+), Anode to (-). The diodes should be rated for the full short-current rating of the power supplies.
The purpose of the diodes is to limit the amount of reverse voltage applied to the output of whichever power supply goes into current-limit first.
Can you connect two power supplies in series to add their voltage? Yes, IF at least one of them is FULLY FLOATING, i.e. not referenced anywhere to ground.
Can you use a PC power supply for one of the sources? Seems very doubtful. Virtually all PC power supplies are GROUND referenced and you would have to be very careful to make a ground-referenced power supply the "lower" (0-12V) half of the series string, with the "upper" half (12-24V) being the fully floating supply.
Since we don't know anything about your unidentified existing printer power supply, this seems rather doubtful.
You are not going to get 20 amps out of that arrangement. You will get 24 volts ok, but the current will be equal to the lower of the two, which will only be around 2 or 3 amps, depending on the PC supply.
Check the label on the PC supply to determine how much current you can draw. It might be as high as 18 amps, but might also be under one amp.