I am a Canadian currently in China, I'm going to visit Hong Kong and then re-enter China

My Chinese visa hasn't expired yet, but its single entry has been used up when I entered China. Assuming entering China from HK requires one count of entry, my current one is then of no use. Must I apply for a new one or is there any way to increase the number of entries on my current one?

If I must apply, is it easier to do it beforehand in China or in HK when I need to re-enter China?
Thanks.