Widgets are often also uploaded to github. So you can clone & customize them from there. Please be aware that typically you have to take care of your own support for the widget.
However the stopwatch widget I see in the marketplace doesn’t appear to be published on github, so that means either you need to ask them for the code, or use a little trick for this. If you look into the widget folder of your project you see the widget. When you unzip the file, you will get the code of the widget.
Sometimes you can also place a feature request on a widget and the developers will provide it for you.
Unfortunately you cannot directly customize a widget in Mendix yourself, however some widgets you might be able to create different circumstances that facilitatie your widgets need by using different contexts.