How to display special characters in Imperavi Redactor?

I'd like to find an easy way to insert special characters in Imperavi's redactor.

In Windows, users can insert a Greek letter such as ß with the keyboard shortcut Alt+225.

But is there a way to have a more user-friendly feature in Redactor to insert such characters, perhaps through a dedicated menu?

Finally found a way with a custom dropdown button :

buttonsCustom: {
            greek: {
                title: 'Special Characters', 
                dropdown: {
                    alpha: {title: 'α',callback: function(obj){obj.insertHtml('α');}},
                    beta: {title: 'β',callback: function(obj){obj.insertHtml('β');}}
                }
            } 
        }