Grouping rows for GetDataCell rows

Maybe I'm missing something easy, but I'm trying to get some rows to group in the Excel import, and I can't get it to work properly. I think the problem may be that the rows are GetDataCells, but I'm not sure. So my question is, will the ExcelOutlineLevel work for GetDataCells, or is it restricted to member trees?

To illustrate visually, I'm trying to get a bunch of rows like this:

To export into Excel grouped like this: