EMS deliveries can usually be tracked in two places: the post office in the country the package is shipped from, and the post office in the country the package is shipped to. So if you have a package from the US to Canada, for example, you could track it using US Postal Service or Canada Post in Deliveries.
Unfortunately it's not possible for us to add China EMS (ems.com.cn) because of the verification code they use. However if the package is shipped from China to another country we support, you should be able to track the package that way.
If you enter an EMS tracking number, Deliveries should automatically detect one of the options for you, if it's a service we support. In some cases you may get better information if you manually change it to the other option.