Ext.define("My.OwnClass",{
    extend: "Ext.tree.Panel',
    ...
    config: {
        clName:'',
        relatedTable:'',
        mode:'',
        ...
    },
    constructor:function(config){
        this.initConfig();
        this.clName = config.clName;
        this.relatedTable = config.relatedTable;
        this.mode = config.mode;
        this.callParent(arguments);
        switch(this.mode){
            case "first":
                this.mon(this, "itemclick", this.makeSmth);
                break;
            case "second":
                this.mon(this, "itemclick", this.makeSmthElse);
                break;
            default:
                Ext.Msg.alert("Wrong mode!");
                break;
        }
    },
    ...
    makeSmth: function(view, record, item, index, e, data){
        var selectedNode = this.getSelectionModel().getSelection();
        ...
    },
    makeSmthElse: function(view, record, item, index, e, data){
        var selectedNode = this.getSelectionModel().getSelection();
    }
}

Ext.define("My.OwnClass.Modified",{
    extend: "My.OwnClass",
    //new constructor
    construstor: {
    ...
    }
}

var superPanel = new My.OwnClass.Modified(...);

superPanel.mon(..);