How can I hook/inject a method of a dll knowing just its RVA address by using C++

Question

I am not sure if hook/inject is the correct term for what I'm actually looking for I want to extend a x86 asm function of a game client using C++ to provide me this functionality:

currently:

lets assume that the target function is something like this

void oldFunction(int opcode, char* data) <- RVA known
{
   //do stuff
}

I want to inject C++ code in it so wherever it gets called by the game client I can extend its functionality on my own .dll resulting into something like this:

void oldFunction(int opcode, char* data)
{
    oldFunctionCalled(opcode, data); // my method
    //do stuff
}

so basically I want to inject my method oldFunctionCalled(opcode) in the oldFunction whose RVA is known