Not sure that I understand your use case:

1. if your AG Grid instances have different column definitions, then the best solution would be to use an external library which handles the export to excel
2. if all your grid instances have the same column definitions and only the dataset(rowdata) is different, then you could: a. collect the data from all grids in a single colletion b. create a "headless" AG Grid instance on the fly, pass the columnDefs & rowData and use the exporting mechanism via the API:

    const gridOptions: GridOptions = {
      columnDefs, // shared column defs
      rowData, // aggregated data from all grid instances
    };

    const grid = new Grid(document.createElement('div'), gridOptions);

    gridOptions.api.exportDataAsExcel({
      sheetName: 'Sheet 1',
      fileName: fileName,
    });

    grid.destroy();