On the way from Berlin to Warszawa, you can safely simply miss Warszawa Zachodnia and in the very unlikely event there will be ticket control between Zachodnia and Centralna you can act surprised and worried that Zachodnia already passed, and I don't think anyone is going to give you a fine. I don't guarantee it, but I'd do it this way without worrying to much. The travel time between Zachodnia and Centralna is just 4 minutes, and everyone on the train is getting up and taking their luggage and piling up in the corridor to get off at Centralna, and even if a ticket inspector would like to check tickets between these stations, it would be very difficult given the short time and crowded space and everyone packing up and generally a big mess.
The other way (Centralna - Zachodnia) the above doesn't apply any more, but you can either try to buy a ticket Warszawa Centralna - Warszawa Zachodnia at the counter in Centralna (should be possible, but I am not sure now), or if that fails just take a bus or another local train (for which you can surely buy tickets locally), or even if its not possible to buy a ticket between these stations for that train, just buy any random local ticket between these stations at the counter and still board that train, so at least you have something if the inspector starts to check tickets there, and even if its the wrong ticket, you can just say you didn't know, and most likely he'll let you go, or in the worst case sell another ticket with some ~10 PLN (less then 3 EUR) surcharge for selling a ticket on the train. Polish ticket inspectors are likely to not give you a full fine, but allow you to buy the right ticket from him with a small ~10 PLN surcharge if you at least have some kind of ticket which one might think might be valid for this train (even if strictly it isn't), or if you are past the station you should get off and you claim you missed the station. They will only give you a full fine if you don't have any ticket at all. You will have to be very unlucky to meet an inspector which would give you a full expensive fine in that situation, most will just sell you the missing ticket (in the very unlikely event that you meet them between Zachodnia and Centralna at all).
Note regarding EIP trains:
However in case your train Warszawa-Kraków is an EIP train, then I would take a different train on the way from Warszawa Centralna to Warszawa Zachodnia, because the rules regarding travelling without a ticket on EIP trains are vastly different then on "normal" (not-EIP) polish trains. First of all it is not allowed to buy the ticket from the train staff, and the fines are vastly higher (650 PLN fee on EIP vs the standard 10 PLN surcharge on any other train in case of the same infraction of boarding the train without the right ticket). I have never traveled on an EIP train yet, so I would be cautious applying any of my previous advice on an EIP train, as the ticket inspectors might be much more strict there.
