That wasn't really what was discussed above, Ishida- claires was having problems with PSOne games saving on a different virtual card than what was assigned. Haven't heard back for awhile so she may have cleared things up on her own.
As for your question, that simply isn't possible. You can copy the saves over to a card or external drive but you cannot set up the virtual card on an external device, nor can you have the games save directly to an external device.
You know, after reading the thread through again, claires never did specify if the games she was trying to have saved were actually PSOne games- they could well have been PS3 games. If that was the case you would have to set up another user on the PS3 for someone else to log into to play the games. The saves for him/ her would be seen only by that profile and won't interfere with another person's progress.