You can probably buy credit online for your Polish SIM card while in Berlin (I have held several pay-as-you-go SIM cards in various countries and it was always possible).
But you should not assume that credit/plans from the same brand are somehow related between countries. Conditions are sometimes very different and shop-bought credit is typically only valid with a SIM card from that country.
If credit for a Polish SIM card is available at all (I doubt it is), it will be a separate product. Technically and commercially, mobile phone networks in different countries are independent from each other, even when they share an international brand like Orange, Vodafone or T-Mobile. Some of them do offer free roaming/integrated offers across several countries (I think it's the case in the Middle East in particular) but it's not a given.
Beyond that, roaming prices – whether on a partner network or not – within the EU are regulated, but still not very cheap for data (see previous questions on this site for more).