Yes, it's possible to add a custom link or button in the workflow action bar in the inbox. Here are the general steps to follow:
- Create a new custom workflow process step by extending the OOTB process step (e.g. "Done" or "Approve").
- In the custom step, add a new property to the process step to define the label for the new button/link.
- Override the execute() method of the custom process step to add the desired functionality when the new button/link is clicked.
- Build and package the new custom workflow process step.
- Deploy the package to your AEM instance.
- Add the new custom process step to the workflow model.
- Open the inbox and you should see the new button/link in the workflow action bar.
To pass workflow session variables to the custom link, you can use the Workflow Data API.