Adding rows to an existing table in Power BI

Maybe the following is a solution for your problem. With Table.InsertRows you can insert an new record at a given index. Not sure if your sort column is dynamic or static. For this to work you would need to add a dynamic indexColumn for the sorting.

See the code below for an example:

let
    // replace the source with your data
    Source = YOURDATA,

    // a list with the record for the new kpi
    newKPI = {[
        Attributes_PnL_Format = "np_totalCost",
        Category = "Revenue by source",
        New_PNL_Name = "Total Cost"
    ]},

    // insert the record at the desired position
    add_newRecord = Table.InsertRows(
        Source, 
        1, // the position you want it to be inserted at
        newKPI // the record
        ),

    // create a index column that can be used for sorting
    add_sortOrder = Table.AddIndexColumn(
        add_newRecord, 
        "SortOrder", 
        0, 
        1, 
        Int64.Type
        )
in
    add_sortOrder