I don't think you can with the stock homescreen setup but you might like to try a home launcher like ADW.launcher which is very customisable and you can add up to 7 screens with customisable number of columns and rows and the option to add shortcuts/folders, multiple widgets per page etc. I recommend that you firstly download an app callled Home Switcher which allows you to switch (!) between your Home (Sony Ericsson) launcher and ADW and then set the new one as default.
