Anaplan allows you to add time period totals from the Time Settings. On the Model Calendar page you have INCLUDE option to add certain time total buckets. But please be cognizant of the fact that it will impact every module in your Model where Time dimension is included.
Another way of doing will be to have a Quarter Total List like a fake time. This can be done if only few modules require the quarter total view. Please pick the best way forward as per your case.
I would shorten the formulas you've suggested to UserSelectionModule.SummarySelection = NewList.Quarters and UserSelectionModule.SummarySelection = NewList.Full Year without necessity to use IF THEN ELSE conditions. I think it is excessive here.