DSS Express is Dahua's own VMS and there can be limitations using 3rd-party cameras with a specific manufacturer's VMS.
Looking at the below comparison table of the different DSS versions it looks like the free DSS Express will allow you to add up to 8 Hikvision cameras using the ONVIF protocol, but this will probably mean that some manufacturer/camera-specific features won't be supported or accessible in the DSS software. To add Hikvision cameras properly using the "Hikvision Protocol" it looks like you need to be using the paid-for DSS Professional software.