Try something like that (haven't tested):
IWorksheet::Parent should return the pointer to IWorkbook which you can use to access all worksheets.
What i generally do=>
Set swDesTable = swModel.GetDesignTable
Set xlApp = GetObject(, "Excel.Application")
Set xlWork = xlApp.ActiveWorkbook
Set xlSheet = xlWork.ActiveSheet
This enables you to access the design table as a normal excel sheet & you can do whatever you want. Like, if you want to access Sheet2, you can use
Set xlSheet = xlWork.Sheets("Sheet2")
Note: Don't forget to add Microsoft Excel 14.0 Object Reference
Hope, this helps...