Possible to extend the VISA to one more entry from China?

Hello,
I am currently in China for business purposes, and I have the M type (one entry). I will stay in total for 2 months. I have recently been informed that I will be for 3 weeks in April in Guangzhou, very close to Hong Kong. I would therefore like to have the possibility to visit Hong Kong for a couple of days, but I need to be able to get back. Do you know if there is any possibility for extending my M type for one more entry?

Thanks in advance,
BR
Marta