Your KLM flight is codeshare with CZ. Which might allow you to
have your check in forwarded to a CZ flight to HK.

You need to buy a ticket from PEK to HK before you depart to PEK.
After deboarding the plane in PEK, look for the INTL Transfers sign
with arrow. Follow it to the first desk to get a boarding pass
for your PEK-HK flight, if you don't already have one.

If you already have a boarding pass, skip the first desk, proceed to
immigration and look for the desk marked INTL Transfers. Present
your passport and boarding pass. They will stamp your passport
with a diamond shaped transit stamp.

Then retrieve your checked baggage. You will be in Terminal 2.
If your flight to HK leaves from T3, then take the bus to T3.