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Add button

First, to add button to the toolbar from the additional set you can use the buttonsAdd option, like this:

$R('#content', {
    buttonsAdd: ['underline', 'line', 'undo']

// the full additional set of buttons, that is:
'line', 'redo', 'undo', 'underline', 'ol', 'ul', 'indent', 'outdent', 'sup', 'sub'

The second way is to create a plugin. See the demo and its code.