You can copy a custom plugin or workflow assembly along with all of the registered steps from one Dynamics 365 organization to another by following the process below:
Connect to the source organization in XrmToolkit.
Use the 'Add Items to Project' menu item (option 1 below) from an 'Assets' project:
Select the plugin from the list in the 'Plugin Configurations' node:
Once the configuration has been downloaded it will reside in the 'Plugin Configurations' folder:
Switch to the destination organization in XrmToolkit:
After connecting to the destination org, open the config file by double clicking on it in the Solution Explorer window. Doing so will open a plugin registration window:
If the plugin has never been registered in the destination org, then you can select the plugin and press the 'Register Plugin and Descendants' menu option. The plugin and it's descendants will now be registered in the destination org:
If the plugin had been registered previously and you are wanting to only register new plugins/steps, then you must first update the plugin assembly in the destination org by using the 'Build And Publish' menu item.
Once the updated plugin assembly has been deployed to the destination org, you can double click on the config file and register any of the new plugins or steps by selecting them and pressing the 'Register Plugin and Descendants from Config':