Is there a way to add actions to the Commonly Used Workflow Actions for SharePoint 2013?
There isn't a checkbox or setting within the action. It just updates over time based on action usage. I'm not sure what the magic number is before it shows up though.
The official answer is if you drag and drop an action 7 times it will add an action to the Common tab. But this is a client process within the browser, so if you loose the cookie where it is stored, it will reset.
Please see the answer from a related question Commonly used actions
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