The classic way to do this is to put your part in an assembly, make a simple part that is slightly larger than your core (typically by an extrude or revolve), then use the Cavity feature to subtract the part from the core base, leaving your core. (You need to be editing the core part in the assembly.)
It is usually considered good practice to scale your original part to allow for the shrinkage.
You will need to have a configuration of your part that shows the original as-cast part, without the later machining.
You can also do something similar in a mult-body part, using the Combine feature instead of a Cavity, but that doesn't make as much sense to me as doing it in the mold assembly.