Why? You have do nothing other than connect the DIP switches to P2. That is all. You don't have to write back to P2 or do any thing else. Just do as you would with a normal port except don't use MOVX with DPTR.
Make sure you write FF to P2 latch(reset state) before you use it as input, as you would for any other port pin. Making DIP SW common to ground eliminates the need for pull up network, if the DIP SW are near the MCU(using internal pullup).