I'm not sure exactly what you are asking, but using the 'constructor' function in your Ext.define will create Getters and Setters for your custom configs, as well as the constructor of the extended class.
Say you have a defined subclass of Ext.Panel and you have a few configs that are new.
Using the constructor will give you the available functions (when referencing MyPanel):
extend: 'Ext.Panel', //extends the Panel class
customConfig: 'some value',
Hope this helps.
PS: You'd have to mark the answer as best