If you adventure in to K2 Management and look through the SmartObject roster you will notice an Azure Active Directory SmartObject called User and another similar SmartObject called UMUser. By looking at these two, specifically the User SMO, you might think that it references AAD but do not be fooled.  
 

The User SMO references the Identity.Identity data table in SQL Azure and while that is a reflection of AAD, the SmartObject itself does not actually look at AAD. That's not a huge concern though as the SMO will return pertinent information with respect to a user account assuming it has proper input, which for this SMO happens to be the account name or Fully Qualified Name and only that.

If you want to use something other than an FQN to get information from AAD, maybe using something like a display name or an email address, then you will have to use the UMUser SmartObject. This SMO will query AAD (SharePoint to if you set the label) against a given property and pull back the information related to the annotated property. To do this within the context of a WorkFlow you will need to reference the Azure Active Directory events and feed them a property value which they will use to create relevant item references.