How to customize the structure of dialog editor of scaffolding?

Like any other CQ widget -- you can customize it. Its just JavaScript/nodes and props.

To become familiar with customizing CQ widgets - I recommend installing the sample widget package as discussed here:

http://dev.day.com/docs/en/cq/current/developing/widgets.html

and going through the exercises in the package. It's actually a good package to install and go through to understand CQ widgets and how to modify them.