Is there any way to hook up another audio device for input on the stock P5 cd players? I wanted to try and hook my laptop up to it for a mp3 player, and since the unit seems to have an optional MD player, can I put the laptop's audio signal there in its place?

 

Simple answer- No.
You could use a device to transmit on FM, or a cassette adaptor, but neither gives decent sound.
I have read a couple of threads where folks proposed cracking the top of the radio open, and soldering into lines. If you are electricly inclinded, that would be the best option.