I found it by myself.

Here is how.

CAUTION: always work on a COPY of your Pogoplug application and on a COPY of the files and folders. Store original application,original files and folders in a safe place so you can reuse them untouched and unmodified.

You work under your OWN responsability.

You must have administrator privileges.

-Create your customised icons and save them as an .icns file anywhere on your Mac.
-Quit Pogoplug Application.
-Right click on Pogoplug application file in your Application folder.
-Select Show Package Content.
-Open "Contents > Resources" folder.
-Drag and drop or copy your customised .icns file in the Resources folder.
-Trash the Application.icns file that you see in the Resource folder and empty the trash.
-Rename your .icns file "Application.icns" without the quotes.
-Close all windows.
-Restart the Finder, relaunch Pogoplug application and ... there you are.

Enjoy...

;o)