adding unsupported instructions

Im developping my own version of a 8051 in VHDL. and Im using Keil software to make test programes. I know how to added my own SFR's, but the problem is If I Want to added a special instructions not supported by the original 8051, the only way I know is modifing manually the hex file generated by the compiler .
I want to make the macro assembler or the C compiler supporting my special instruction by specifing the new opcodes . is this possible? please give me your advice I will be very thinkful.

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