Easily done. You need a itemeditor program for tibia which you can download I believe in otland's download section...
Once downloaded , do the following;
1. Open the program , Click 'File' > Load OTB > *and browse to your items.otb that should be located in folder 'items' in your servers data.
2. When it loads, open up the 'Containers' section [speaking of a box..] , find the box you are talking about (find it by it's ID number), and take a look at the attributes of it. The attribute 'Not Moveable' will be checked.
3. Uncheck it. [configure everything how you want] , press Save As and replace your old items.otb with the one you just made.
4. Restart your server , and it should work.
tell me if you have any issues further , i'll be glad to help