How to add an additional wrapper div within a Views row?
This could be done in a couple of ways depending on your requirement.


If you want to add a wrapper div to a particular row: (Note: This will work if you are displaying fields in the view)


Click on that field.
Under style settings select the HTML element, and choose DIV. 
You can also add the class to this div with the "Create a CSS class" option.

If you want to add a wrapper to all the rows: (NOTE: This will only work for D7)


The best way to do this is via a views template. Click on the "Advanced" section on the right hand side.
Click on the "Theme information"

Select "Views row style output"
Copy the template content.
Create a template with the name provided against "Row style output" link in the screenshot above. Paste the content into it.
Add an extra div element in the template directly.
Note: The template overrides are provided from "generic" to specific" if you go from left to right. E.g. views-view-fields.tpl.php would get applied to all the rows of all the views, and the specificity of the templates goes on increasing as you go right of the "Theme information".