You can add content to code plugins as well. You just have to create a content folder for your plugin, you can choose to create one when setting up your plugin in the setup wizard, or you can add the content folder manually to the plugin’s directory and add "CanContainContent": true, to the plugin’s .uplugin file.
You can now migrate any blueprints or assets from your project into the plugin.
For c++ classes you can move them into your plugin’s source folder. You will probably need to do a bit of refactoring to get things hooked up correctly.
The wiki also has a few tips on migrating c++: