By default in O365, SPD 2013 reusable workflows are not supported running the Workflow Manager Engine.
Unlike previous versions, SharePoint 2013 does not support workflows that are scoped to a content type (SPContentType). However, the messaging infrastructure is extensible, so it can support any arbitrary scope. As a developer, you can set the EventSourceId property on a given WorkflowSubscription instance to any guid. SharePoint 2013 workflow fundamentals
As of today, the only Nintex workflow types listed are List and Site http://help.nintex.com/en-US/O365/#O365WorkFlow/Creating workflows/Workflow types.htm
But the workflow fundamentals from msdn states that it is possible if you handle the association yourself. So I hope to see this as a feature.
I found that it is currently in uservoice that you can vote for as I have. Also it is currently under review! Ability to Create Reusable Worfklows – Customer Feedback for Nintex