So the car already has Polish OC but you want to add short-term AC? Some insurers may allow you to buy one-month AC, but the condition may be that the car is OC insured with them. Hence, your best chance is the current OC insurer. Check this article: azpolisa.pl/Ubezpieczenia/OC-na-miesiac-AC-na-miesiac-Krotkoterminowe-polisy.html