You could even OR the External Memory outputs with Program Memory outputs and write self-modifying code because MOVX and MOVC would operate on the same memory. (some consider this an advantage... ;)
I know what you mean, but I do not think Arron will.

You can change the Harvard architecture (separate code and data memory) to vonNeuman (same memory for both) by negative ORing !RD and !PSEN together with a '08 gate. This way it is possible to use a NVRAM for code that can be modified. If you do so, be careful, you can easily get in trouble and with your statement of being a beginner, I would strongly advise against it.

Erik