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?
1
Turn on the PS3 by pressing the center "PlayStation" button on the PS3 Dual Shock controller.
2
Insert the USB Flash drive into a USB port on the front of the PS3.
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3
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.
4
Select the game file that you want to install and press the “triangle” button on the PS3 Dual Shock controller.
5
Select the “copy” option from the menu. The options menu will close after selecting the “copy” option.
6
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! )