As long as you have not moved your selection or changed it in any way, you can add to the original selection area by holding down the Shift key. Simply choose a selection tool, hold down the Shift key, and draw with an additional selection as you would normally.
The small plus sign inside the selection cursor indicates that instead of creating a new selection, you are adding to an existing selection. Feel free to mix and match selection tools. Even if the original selection was created using the rectangle selection tool, you can add to it with the Oval, Magic Wand, Polygon, or Lasso tools.
Adding to a rectangle selection using the oval tool
Keep adding to a selection as many times as you want until you select the entire desired area. You can also add a selection area that is completely separate from the original.
Adding a separate selection to the original