While it isn't possible to integrate third party components in the toolbox you can use them in a Radzen application. This forum thread shows how. Basically you need to:
- Add the nuget package to the .csproj file (use visual studio or command line)
- Import the required namespaces (can be done from the application settings)
- Include the required JS and CSS (can also be done from the application settings)
Radzen 3.0 would infer all components imported in an application and make them available in the toolbox.