Symptoms

Unable to pull user data.
 

Diagnoses

You can use the System URM SmartObjects UMUser, UMGroup, UMRole, UMLabel, which target the Appit server's Identity tables that cache SP and AAD information (and pass in SP as the label to limit your search). There is also a 'SharePoint Users' SmartObject as part of the SharePoint User Functions Service broker. When you appify a list, it also creates an AAD SmartObject with the name of your site collection / site / list that can query users as well.

Regarding custom AD properties, the out of the box brokers only handle default properties, but you can expose custom AD properties via a web service and then connect to that using the 'Endpoints WCF' or 'Endpoints WebService' brokers.
 

Resolution

You can find the aforementioned SmartObjects by searching for 'user' in the 'Execute a SmartObject' section of K2 Management, which can be found via Site Contents -> K2 Appit for SharePoint.

Details for the Endpoints brokers can be found here.