but I am unsure if in the company accounts this can be declared as a cost or not.
Yes, but the calculations aren't quite 1zł to ZUS = 1zł in costs. If you don't know what you're doing, get an accountant - it'll cost you between 100-150zł+VAT a month for peace of mind, and you just throw your bank statements / receipts to the accountant at the end of the month and they'll tell you what to pay and when.