Hi Bastien,
The only way I got around this is to first create a report with the correct amount of columns with the to dimensions. Even if it expands to 20 columns, you can delete the excess and they wont re-expand unless to redefine the report in the report building.
Second setup was to setup in a separate sheet the members you want to show in each of the columns. (using excel formulas based off current view). In this way you can create a similar concept as the structure in BEx Query designer.
Last step was to edit the EPMOLAPDimension formals in the report columns (sorry I can't remember offhand exactly the formula).
Where it would define the time member, replace with a formula point to the time on the second sheet. as long as its in the correct format i.e. "[TIME].[2016.001]" then it should retain it. Do the same for the account, then copy it across all columns.
This process is a bit of trail and error, but it did work. If you change the report editor, it will probably re-expand members over your formulas. However, once locked and saved, it has prove to be stable and working fine.
Tim