Poland is member of the Single Euro Payments Area (SEPA), so NS should be able to accept a Polish bank account for periodic payments. However, you can't make your first payment online... Perhaps you can try at a manned station to arrange it face to face? Pay with cash or credit card for the first month and register your Polish bank account for future payments?

Probably you'll also need to provide a Dutch address…

See also: 

Good luck!

So no issue with transfering different currency to PLN then? I don’t mind going to the station in Amsterdam to get it done, I just don’t want to get overcharged (I had friends who somehow got so) or not be able to cancel it from my card easy.

You can pay everywhere with your Polish bank card, it doesn't matter that your bank account isn't in Euro, the bank will make the conversion. I would give it a try at Amsterdam Centraal. But I can't promise it will work out for you.…

In order to cancel the subscription, you may need to go to an OV Chip machine, or change to Flex Basis and stop using the OV Chip card

You don’t think I can cancel the subscription online like by email or something? Going in person to a machine seems like an inconvenient hassle. I asked customer service twice but they gave me conflicting answers...

NS flex basic is free. With NS flex you can travel (in the Netherlands) and pay afterwards. You can change your season ticket online from NS weekend to NS basic. This change takes effect immediately (if your weekend subscription started more than a month ago).

This can be done on your personal page (internet only, not app). Customer service can also arrange this via chat or by telephone.

You don't have to go to a vending machine.  However, you will continue to receive an invoice every month.  If there is no travel, the NS flex basic invoice is €0.

If you want to travel on balance (which means putting money on your OV chip card in order to travel), you must go to the machine.