As far as I know there aren't any homebrew Ad-Hoc WiFi file transfer programs available as yet.
The only program I know of that resembles what you want to do is called the "TedZero Browser", it allows you to explore your PC files and PSP files and copy between the two using WiFi Internet (Infrastructure Mode).
You can check out psphacking101.com Episode 8 for a video tutorial of how to use it!
Cheers,
Fox
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