In the new 365 Addin it is not possible to select an existing subset instead of manually marking elements for a dimension.
One has to go through the entire hierarchy to select the elements which should be included in rows or columns (or even POV). Existing subsets, which are used in the rest of the ecosphere (web spreadsheets, database modeler etc.) are not accessible.
Thank you for your great idea. While we cannot commit to a specific target version, we will consider adding it to our future internal backlog. If you can add further information about the context of this feature, please add it here so we can evaluate it more fully.