No, it is not possible to change the maximum number of harmonic orders when calculating THD. On the WT210/WT230, the THD values are calculated based on 50 orders.
To calculate THD on the WT210/WT230, there are two methods that can be used: IEC and CSA. These methods are described in the User's Manual on Page 7-5 of IM 760401-01E.
Refer to the attached JPEG for quick reference to the formula.
Measure characteristics of devices that generate, transform or consume electricity. Also called power meters or wattmeters, these devices measure parameters such as true power (watts), power factor, harmonics, and efficiency.