yes u can. i have put in a ps1 game and play successfully
without any problems. you will need to have your ps1 game ripped to
your pc, then use programs such as popstationgui and convert it you
will can a file called eboot.pbp
put this eboot.pbp into a folder. for example < game
name > then put this whole folder into the games folder in your
psp instead of ISO. you should be able to play it.
you can adjust screen sizes and buttons by pressing home.