copying an excel worksheet to an existing xls template file

I have a code which populates an excel worksheet, and saves it as (using saveas method of the worksheet) to a a new file. what I want now is how to save the constructed worksheet to an existing excel template (xls). I am using vb.net, and here is my code. I ppreciat your help. Thanks
           
            Dim excel As New Microsoft.Office.Interop.Excel.Application


            Dim wb As Microsoft.Office.Interop.Excel.Workbook = excel.Workbooks.Add
            Dim ws As Microsoft.Office.Interop.Excel.Worksheet
            ws = CType(wb.Worksheets.Item(1), Worksheet)
            sFileName = txtFeedFile.Text & ".xls"

            'DS is a dataset containing the table that I want to export to excel
            dtMLI = DS.Tables(0)

            'Populate Excel spreadsheet
            Dim dc As System.Data.DataColumn
            Dim dr As System.Data.DataRow
            Dim colIndex As Integer = 0
            Dim rowIndex As Integer = 0

            For Each dc In dtMLI.Columns
                colIndex = colIndex + 1
                ws.Cells(1, colIndex) = dc.ColumnName.ToString
            Next
            For Each dr In dtMLI.Rows
                rowIndex = rowIndex + 1
                colIndex = 0

                For Each dc In dtMLI.Columns
                    colIndex = colIndex + 1
                    ws.Cells(rowIndex + 1, colIndex) = dr(dc.ColumnName).ToString
                Next
            Next

            'Save excel MLI file
            ws.SaveAs(sLocation & "MLI_DEPOSIT_" & sFileName)