This is available for Planners.
A planner selects a moderator for a committee from the list of graders already added to the committee. If graders have not been added to the test as contributors, follow the instructions here: Adding contributors to a test
Assign a moderator
To assign a moderator, make sure there are at least two graders in the committee as moderators can not set any marks or grades. There can be only one moderator per committee. Changing moderators can be done by the Planner up until final grading is confirmed.
1. Open the test in the Deliver module and scroll down to Committee. Click Manage.
2. To add graders to the committee, select a committee > click Edit graders.
3. Once the graders have been assigned to each committee, select a committee > select Edit moderator. 
4. Select a moderator for the committee. To notify the moderator, click Notify Moderator. Click Update to save your changes. 
5. The selected moderator's name will appear under the Moderator column in the Committee overview page. To exit, on the upper-right corner, click Close.