PSB (Public Security Bureau)

Hi there, I've been granted a visa for 5 days, from the 23rd - 27th May, flying to Beijing. Ideally, I'd like to stay for 3 more days to do some sightseeing - Great Wall of China etc. However I understand I cant sort this beforehand in the UK, at only at a PSB (Public Security Bureau) in China? The trouble is I will be too busy to go in person over the five days, but only at the end of our contract. And then if the PSB (Public Security Bureau) take 7 days to reach a decision, it seems this would be too late in any case? Also where is/are the PSB's. Is there any way around this? Best, Dom