I don't think you can.
would be dumb of PSN though if u could...
---------- Post added at 12:32 AM ---------- Previous post was at 12:29 AM ----------
edit: Maybe this works?
Turn on the PS3 by pressing the center "PlayStation" button on the PS3 Dual Shock controller.
Insert the USB Flash drive into a USB port on the front of the PS3.
USB - 'The Basics'
Simple Tutorial - USB Nuts & Bolts 4 Page Paper - Free PDF Download
Navigate through the Xmedia bar until you reach the “games” icon. Scroll down and select the “USB Flash Disk” option with the "X" button on the controller.
Select the game file that you want to install and press the “triangle” button on the PS3 Dual Shock controller.
Select the “copy” option from the menu. The options menu will close after selecting the “copy” option.
Highlight the game and press the “X” button on the controller to select it.
Read more: How to Install PSN Games From a USB | eHow.com How to Install PSN Games From a USB | eHow.com
(found on internet, didnt really read, hope its usefull! )