Adding new row with subtotals to the table.

You can try with steps below.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUVKA4kQgtjAAEoZQbKAUq4OqJAmIjTCVGCEpSQbJGGCqMUZSkwIWhWCQCl1DpdhYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ID = _t, #"Snaphot Date" = _t, #"Key (Id + Snaphsot Date)" = _t, #"Business Unit" = _t, Allocation = _t, #"Allocation sum for induvidual" = _t, FTE = _t, #"Diff FTE vs Allocation" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Allocation", Int64.Type}, {"Allocation sum for induvidual", Int64.Type}, {"FTE", Int64.Type}, {"Diff FTE vs Allocation", Int64.Type}}),
    #"Grouped Rows" = Table.Combine ( Table.Group(#"Changed Type", {"ID"}, {{"A", each 
            let Transposed1 = Table.Transpose(_),
                Totals = Table.AddColumn(Transposed1, "Custom", each try List.Sum(Record.ToList(_)) otherwise ""),
                Transposed2 = Table.RenameColumns (Table.Transpose(Totals), 
                List.Zip ({Table.ColumnNames(Table.Transpose(Totals)), Table.ColumnNames(_)})) 
            in  
            Table.ReplaceValue(Transposed2,"","Not Allocated",Replacer.ReplaceValue, 
            List.Select( Table.ColumnNames( Transposed2), each _="Business Unit")), type 
            table}})[A])
in
    #"Grouped Rows"

The result: