Need a device that can take single input and covert to a keyboard key press on USB

I gather that detecting a keypress is not the real purpose, but that you want to detect the digital signal from the transistor one way or another.

There are lots of modules with FTDI's FT245R USB interface, which allows you to connect up to 8 digital I/Os. For example this one costs only 9 dollar (and you may even find cheaper ones).

If you want to stick to the keypress then completely taking apart the keyboard is a solution. Most keyboards have only a small PCB with the controller, and a connector to the actual keyboard matrix. Find out which two pins define the P key, and connect the transistor between those pins. Then you only need that small PCB, and you can throw away the rest.

What you're looking for is a "keyboard encoder" or "keyboard emulator", such as these.

They also used to use the term "keyboard wedge" for this sort of thing, but nowadays that term seems to be used exclusively in the context of point-of-sale barcode and credit card scanners.

I'd personally attack this with an Arduino. Simple, tons of existing support from the community and cheap. You could extend the functionality quickly and easily if you so chose to.

Here's a relevant link to a rough schematic, code, projects similiar, etc: PracticalArduino