Good afternoon Bartosz256. Welcome to the NS Community.

Sadly, it will not be possible for the finders to hold onto your backpack a day longer. It's a pretty strict process with no room for "requests". Your bag will be sent to a location closest to where it was found and you'll be able to collect it there.
Once you can do so, you'll be informed about it by email.

I lost my backpack on the Breda Roosendaal route, there is no point at the station in Roosendaal. so he'll probably send my backpack to Utrecht, right? 

That's correct.

so I can easily pick up my backpack in Utrecht on Saturday? the backpack won't go anywhere else? 

I'm not sure on which day you'll be able to do so. As stated before, you'll be notified per email when you can do so. The backpack won't go anywhere else.

The found item may have been handed in in Roosendaal or at another station. If there is no service desk, it will be in a safe and you cannot pick it up at that station.After 5 days, objects are sent to the Lost and Found Office in Utrecht. This is not at Utrecht station. If your item ends up here, we can only ship it to you.-PICK-UP IN UTRECHT IS THEREFORE NOT POSSIBLE-The shipping costs are €15 for standard shipping within the Netherlands. For larger packages, these costs can increase significantly.

great, thank you very much for your help