you cant, unless you root your phone and install a custom ROM which will then automatically do this if you partition your SD card. there are numerous threads on rooting and custom ROMs how to's. hope this helps.
i already root my phone, but as far as I can see, there is only one partition on sdCard....so? any advices?
you'll probably have to do some digging on google for more accurate help, but i used a partition manager to create an EXT3 partition on my card.
