Use Add to Selection to add the new selection to the currently selected objects.
Click Add to Selection
in the toolbar, and select several objects. Click one of the selection tools and select a few different objects. The new selection is added the old one.
Choose Edit > Select Mode > Add to Selection.
Place several objects on the canvas.
Use Box Select (or another selection tool) to select some of these objects.
Click Add to Selection
.
Using Box Select , select some different objects.
The new selection is added to your old selection.