Log into your Ikariam account and make sure your warehouse is upgraded to a good level (for example level 4 or 5). Change the date on your system. Presuming your warehouse can hold the sufficient amount, your supplies will increase. Note: Avoid trying this if you have an active building queue; your supplies will just reset. Before you change your system date back to normal, close the Ikariam account or else your supplies will go to zero or minus.