Hi James,

You are correct, the appraisal does lock the group of assigned users once the appraisal is activated. If there are new hires you can create copy the appraisal using the clone button, bump out the stage due dates, and then assign the new users to that appraisal. 

EDIT: Also note that in Totara v2.7, there will be changes coming in the way appraisals behave as noted here: https://totara.community/mod/forum/discuss.php?d=9563

Cheers,

Lori