The original source code is lost when the hex file is produced. The hex version is only the binary machine instructions, all the variable names, structures, functions and formatting are lost forever.
The best you could possibly manage is for someone to reverse engineer it and make a new program with equivalent functionality. Before anyone would undertake such a laborius task you would have to give some idea of how complicated the program is , for example which PIC is it for, how big the program is and what it's exact function was.
Brian.