As I understand, this is not a question on control development, so, transition control or not, this is irrelevant.
You don't have to do anything special. When you successfully develop a control and build a solution with this control, it will appear in the toolbox. However, you need to reference the assembly with a control by an assembly where you want to use it (if they are separate assemblies, of course). Besides, you need to know how to reference a newly developed control in your XAML where you use it. It does not really require the use of the toolbox. Basically, you just need to add an additional namespace to your XAML and name if in a certain way. You don't really need to remember the namespace naming conventions, because Visual Studio Intellisense
will help you to choose the namespace based on the assemblies you have already referenced.
You can find some basic code sample, for example, here: http://www.c-sharpcorner.com/uploadfile/mahesh/user-control-in-wpf/
This CodeProject article can also be useful: How to Create a WPF User Control & Use It in a WPF Application ( C# )