How to override a function of extjs in CQ/AEM?

I'm using selection Xtype and checkbox type property (CQ.form.Selection) to create a checkbox group in CQ5 (API docs at http://docs.adobe.com/docs/en/cq/5-6/widgets-api/index.html?class=CQ.form.Selection). But I want to override setValue, getValue and validate functions of it in order to meet my requirement. How can I do that via JCR node declaration ?

many thanks and appreciate.