Visual Studio Team Services doesn't allow you to customize the process template and/or Work Item definitions. That's why witadmin isn't working.
However, the Portfolio Management features are available in Team Services. If you are using the Scrum Process Template you will have Features that are broken down into a backlog of PBI's, and the PBI's are broken down into Tasks. You do not have the ability in Team Services to add additional layers like you do in TFS on-premise.