To create a workflow using placeholders, you will need to create a new contact with a placeholder name, i.e. Designer Placeholder, and provide a fake email address. Create as many unique placeholders as you need for your workflow(s).
Next, create your workflow by adding the placeholder contact(s) where applicable.
When you are ready to route out a proof using that workflow, simply replace the placeholders by adding the correct contact to the placeholder tier, and removing the placeholder(s). This gives you the ability to create workflows ahead of time without having to know the specific reviewers until the proof is routed for review.