Mockups for Desktop supports the concept of Symbols (also called templates, master pages, custom components, widgets, etc.). Please read our symbols documentation to learn more about this powerful feature.
Mockups also provides the ability to add your own custom icons which you can use with any of the controls that support icons. See the custom icons documentation to learn how.
If instead you have a very special control that’s specific to your project, we suggest you take a look at the import image feature, which will even let you “sketch-ize” your image to make it fit with the rest of the app.
For complex controls that are comprised of simple mockup controls, Symbols are ideal for this.
You may also be interested in Mockups To Go, a user-contributed collection of ready-to-use UI components and design patterns built using Balsamiq Mockups.