"Supposing i declare the sfr in my VHDL code"

You don't declare it - you implement it in the VHDL.

The SFR declaration is used in user code by people writing software to run on the device; you are making the hardware which is addressed by that software!
Therefore, you need to examine how the 8051 hardware accesses its other ports - and do that!

"8051 understand that there are 2 new ports appended to it."

It doesn't understand!
It just receives an opcode which references a certain address, and it accesses that address; if there is port hardware at that address, the port hardware will be accessed; if there isn't anything at that address - well, who knows what'll happen (it is mentioned in the datasheet, but I can't find it just at the moment)