If you add a part after the "Parts Costing Completed" option has been selected then this part will not be transferred across to Navigator. You will need to go into Navigator and manually add it to the job.
This is fine providing you re-authorise the costs. However, please be aware that it will not remove this job line on Navigator. It will un-authorise the previous job line and supercede the new labour costs to a new job line. Below is an example: