No, as you've stated the row number is not part of the table, it's only there to help you in case you need to index the table rows and thus will always go from 1 to the number of rows in the table.
If you want a special identifier for each row, then you'll have to explicitly add that identifier (which you can call RowNumber if you wish) as a table variable.
mergedtable.RowNumber = [1:height(firsttable), 1:height(secondtable)].';
