I created a Member Property on Time.Day for "WeekOfYear", using the column on
the underlying time dimension table.

I was able to expose this Member Property by using the "Property Fields"
menu option on the PivotTable menu, so it shows up next to Day.

Now, I want to subtotal on WeekOfYear instead of Day or Month. How do I do
this in Excel?

Is there no way, meaning I should go back and make a second Time dimension
(Time2) which goes Year->Week->Day ?