Hi Diego,
If entering Hong Kong airspace from China you will generally be transferred to Hong Kong at a metric level, depending on the entry point and Hong Kong ATC will transition you to an imperial level, if necessary. Similarly, leaving Hong Kong for China you will generally be climbed to a metric level, depending on route, by Hong Kong ATC prior to handover to Chinese ATC.
There are some areas of potential confusion e.g. overflying traffic landing Shenzhen will be on Shenzhen QNH whilst departing traffic or overflying traffic into China will be on 1013.2, but this should not bother you as it is an ATC responsibility.
You will be required to meet strict level requirements out of Hong Kong before entering Chinese airspace, which will generally only be a problem if you are flying an A340-300 at MTOW.
Have a look here for Hong Kong requirements:
Hong Kong Air Traffic Control
I can't help you with altimeter setting procedures procedures in China, but so long as you can translate metric levels into imperial levels correctly and vice versa you should be okay.
Hope that helps.
On the beach (ex-VHHH)