To add availability on behalf of hiring managers, you can navigate to their landing page under their staff profile. To do this, login to the child account that the hiring manager is on and click on Settings > Staff:
Under the staff list, click the "View" button next to the hiring manager you want to add availability for:
Under that staff's profile, click on the "Interview Availability" view:
In the Interview Availability section, so long as the hiring manager had their landing page set up, you'll see the landing page link:
Click on that link to go to the hiring managers availability landing page:
Select the dates from the calendar on the left that you want to add times for and then click the button on the right to "Add New Times". Enter the times you want to add and those will be included on the hiring manager's availability for interviews.