Is it possible to have an alert based on specific user actions?
Having an alert message appear in the form of a pop-up window to be displayed in the user interface would require a customization.
There is another possible solution but the notification would be after the action has been performed. For example, instead of showing a pop-up alert when users submit time to a task that has 0 planned hours, you could create an email notification to alert the project manager or time approver when a user submits time for a task that has 0 planned hours.
For more information about email notifications, see the Changepoint Adminstration online help or guide.