This is a little trick I know, it's probably old news, but I'll throw it out there.
If you want to transfer any file between 2 PSP's (any firmware, custom or official), you can do this:
1) store the file(s) in a .rar file
2) create a .jpg file that can be used to identify what is stored in the rar
3) concatinate the .jpg and the .rar file together into a new jpeg file from the command line
4) transfer the concatinated picture via adhoc between 2 PSP's.
If you open the picture in WinRAR (or any unrar utility) it will act like the RAR archive, but if you open it in any image viewer, it treats it like the jpeg image.
This is also useful for hiding files, but we won't get into that.