Buy Psn prepaid card with Psn credit?

Can I buy a code for 20 euro psn credit in the PS Store or anywhere else, for example with 50 euro psn credit? So that I only have 30 euro left, but a code for 20 euro that I can redeem on another account?

Usually that shouldn't work.

You could try returning the card at the store with the receipt, but PSN cards usually work so that the code can be activated and redeemed after purchase. So no undo.