Yes, it is specific to the FBW A32NX add-on. You can change fuel mid-flight. You just need to use the EFB to do it. (You also need to use the EFB to change the fuel level on the ground.) On the ground, you have the option of instant, fast, or real refueling time on the EFB refueling page. In mid-flight, it will only be instant.
Setting or changing your payload is similar, except that it is done through the MCDU. The loading speed for payload can be selected in the EFB options.