Yes. Mockups for Desktop supports the concept of Symbols, templates, master pages, custom components, or widgets. Please read our symbols documentation to use this powerful feature.
Mockups also provides the ability to add your own custom icons to the app, so that you can use them when you use any of the controls that support icons. See the custom icons documentation to use this feature.
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. We would also appreciate a screenshot or two of your special controls if you can share them, we might be able to add a more general form of it easily.
For controls that are really just groups of simple mockup controls, see this article.
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.