You are wrong. PSP games DO come in PKG format the same way PSN games are. If its sent to your PS3 its in PKG format so as you can download PSP games to your PS3 they are in PKG format.
I think everything from PSN is sent in PKG format. Even games from PSN directly to the PSP are in PKG format first. It's like $onys ZIP format. They always send a PKG then it gets extracted when its finished downloading.
Now you your question, if its PKG then just install it like any other PKG.
Okay, managed to get them on the PS3 by acting like they were just any other package. What hasn't worked is getting them onto the Vita. The PS3 says it's unrecognized USB Device, and the Vita says to update my PS3... Any thoughts on CFW's that support the Vita?