Is it possible to add any 20 series GPU card to Precision 3620?

The 20 series GPU would require an 8-pin power connector, which your workstation does not have.  It's not possible to add any 20 series without changing the power supply.

You can use a 24-pin to 8-pin adapter cable and upgrading the power supply to your needs.

But considering the cost of changing the psu, an alternative would be a RTX A2000 , that would cost ( used ) way less than a 20xx + psu while performing like a rtx 3060 and consuming just 70W ( so no need to change anything )