I spent some time researching about this topic but unfortunately I didn't find nothing related and even related with plugins, so I'm thinking that is not so common to integrate Zenpen to Confluence (and as Confluence has its own editor - https://confluence.atlassian.com/display/DOC/Quick+Reference+Guide+for+the+Confluence+Editor), but anyway if you think that this would be a good feature and also helpful to Confluence then I would like to suggest you to raise a feature request at our https://jira.atlassian.com.
For more information on how Atlassian implements new features and improvements please check the following article:
Hope this helps to clarify.
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