Use Add to Selection
to add the new selection to the currently selected area.
Add an area to a Selection
Place an image on the Canvas.
Use Box Select (or another selection tool) to select an area.
Click Add to Selection .
Using Box Select , select a different area.
The new selection is added to your old selection.
Shortcuts
Draw Tab, Select; Paint Tab, Select:
Keys: Alt, D, SD; Alt, P, SD