How can I convert an unplanned resource to planned?

Select the task in the Manage Project Plan page, then select the Manage Resources for Selected Task action. In the Manage Resources window, select the Unplanned option for the resource.

If you're planning by periods, enter the values for the amount and planning periods in the Convert Resource to Planned secondary window.

If you're not planning by periods, then the actual quantity is automatically copied to the planned quantity for the resource.