According to this article it's possible, though probably not really nice. You will have to deploy your Workflow as a SharePoint App. Which means you will only be able to call the webservices.
Another option which might be worth looking at is Nintex, which has a module for office 365 as well. Nintex has alot of useful functions, though I don't know how good the office 365 version is. You would have to check whether it suits your requirements.