yes, it is possible to do as long as they are both just standard microcontrollers.
first you will have to look up the datasheet for the original microcontroller.
then get the datasheet for your pic. you will have to see where each pin is going to and what it is.
anything that is going to a digital pin must go to a digital pin on your pic, anything thats going to vcc must go to vcc on your pic etc..
its not that hard a task but there is a fair amount of research involved and it takes up a fair bit of time.