How do I add new employees to an existing objectives cycle?
Unlike the performance-review and 360-cycles, people don't need to get added to objective cycles by HR. Either you have objectives in the current objective cycle (by creating them yourself, or via your manager) or you don't.
New people can create objectives as long as the review cycle is "open to edit". If the objectives in a cycle cannot be edited anymore, you could either enter their objectives as the HR manager, or extend the deadlines and tell the new employee to get it done on a day, and then set the dates back to what they were before, making the objectives cycle non-editable again.