You can easily do this with COM automation.
From memory the steps you should take.
1. Reference the Excel Type library (project -> references)
2. Declare a variable of excel.application
3. Create the excel.application object
4. Use the excel.application to open the excel file
5. Using the cells collection or the range object insert(append) your new data..
(if you wanted to do this on a new spreadsheet (Not workbook) - you would have needed to use the "activeworkbook" object and added a New spreadsheet to it...
(to find out what the code would look like in Excel (and you can cut and paste this into VB) just go to tools and "record a macro".. In the macro that you are recording open your xls file, do what you would like your vb app to do, stop recording your macro, then take a look at it.. at this point add the code to a vb project before step 4 and you are done...)
HTH
Rob
PS code might look something like...
this opens an existing file adds a new worksheet (before the current one (default)
in that new sheet it adds a value of test on row 1 col 3 and then saves it as something new before shutting down excel...
Code:
Dim x As Excel.Application
Set x = New Excel.Application
x.Workbooks.Open "C:\Jeremy.xls"
x.Worksheets.Add
x.Cells(1, 3) = "test"
x.ActiveWorkbook.SaveAs "C:\NewJeremy.xls"
x.Quit
Set x = Nothing