Hi Jekatherina,
I believe this is solved in the most recent version of workspace: https://www.ibm.com/docs/en/planning-analytics/2.0.0?topic=20103-change-member-type-consolidated-in-dimension-editor.
In saying that I have not tried it myself as I have not upgraded to this version yet but I watching the IBM PA Q2 product update and they showed creating elements as consolidated type only with no leaves under it (previously it would just default to numeric/string type as you indicated).
If anyone has access to the version of workspace and confirm, let us know!
Ronan