As a Manager, you can add offline for your users. Here are the steps:
1) Go to "Track Time" page.
2) Select a specific project in the project drop-down at the top right. This would be the project you are adding time to. (Please do not select "All Projects")
3) Choose the target user and target date.
4) Choose "Add Time (as Manager)" in "Actions" drop-down.
5) A pop-up browser window will come up. You can just follow the instructions to add time for the selected user.
Here is an illustration.