The way to generate scrollbars is to put a div around your content, style it with a height in pixels and overflow-y:scroll

Where exactly you put the div is up to you. If you're going to make a special container, then put the div in there. If you just want it within a text/html module, put it around the text in the editor's source view. If you want it around any other module, you can put it in the header and footer boxes that are in the module settings page.

Alternatively, you can inject your module into zldnn's module wrapper and apply the scroll settings that is has.

Rob