Visa entry extension in beijing from single entry to double entry.

Hello,

I am in Beijing these days on a tourist visa, single entry valid for 30 days. here i am staying with my friend in his apartment.
Now i intend to go to Hongkong, and then return back to beijing, but i guess, i won t be able to do so with my current visa.

Please advice me, how can i get another entry visa for china, after i return from Hongkong to catch my flight back to Milan from beijing.

also, please give me the address of the office, which you mentioned above.

Appreciate the efforts.

Thanks.
RG