Yes, you can request to extend an assignment for a candidate already working with your company.
To do this, go to the Day Book >> Click the Request to Extend Assignment icon against the candidate.
Plot the shift extension as below (the black ones are existing and the pink is the extension). Click Done.
Enter your comments (reason for extension) and click Submit.
You will get a dialog box like the one below.
The assignment extension petition is sent to the agency at this point. It requires their approval for this extension shift(s) to be confirmed.