I was working with SketchFlow in Microsoft Exression Blend.  There was a DragDropItem behavior that I wanted to add to my behaviors.  From the Microsoft Expression Blend Help, here is how you go about adding a behavior created by someone else.  Here the link to the information.

To add interactivity to your application without having to write code, you can drag a behavior from the Assets panel onto an object in your application and then set the properties of the behavior. You can also use behaviors that are created by people in the Microsoft Expression Blend community. For example, you can download custom behaviors from the Microsoft Expression Gallery. If you want to use a behavior that was created by someone else, add a reference to the .dll file or project that contains the behavior. This will make the behavior available in the Assets panel.

Note:

Developers can use the new Expression Blend Software Development Kit (SDK) to create custom behaviors. To view the SDK documentation, click Expression Blend SDK User Guide on the Help menu.

To add a behavior that was created by someone else

  1. Add a reference to the .dll file or project that contains the custom behavior.   From the Project Window, right mouse click on the Project to get the project menu, click Add Reference... to add an assembly (.dll or .exe) to your project.

  2. Build your project (CTRL+SHIFT+B or F5).

  3. Open the Assets panel to view the custom behavior in the Behaviors category.

    You can now drag the behavior onto objects in your application.

     

To make a custom behavior always show up in the Assets panel

If you download an assembly that contains multiple behaviors that you think you will use frequently, you can register the assembly to make the behaviors available to any project you create without having to add a reference.

  1. Do one of the following (Note: the documentation registry path says HKEY_CURRENT_USER\Software\...,  I found it in HKEY_LOCAL_MACHINE\Software... [I am running on Vista]): 

    • For a behavior that is written for Microsoft Silverlight, create a registry subkey in \HKEY_CURRENT_USER\Software\Microsoft\Expression\Blend\v3.0\Toolbox\Silverlight\v3.0 that is named the same as the assembly namespace.

    • For a behavior that is written for Windows Presentation Foundation (WPF), create a registry subkey in \HKEY_CURRENT_USER\Software\Microsoft\Expression\Blend\v3.0\Toolbox\WPF\v3.0 that is named the same as the assembly namespace.

  2. In the subkey, create a default string value and set it to the path of your assembly.