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.
A Digital Power Analyzer is an instrument for research & development, design validation, and manufacturing test used to accurately measure electrical power characteristics.