I'm not entirely certain what problem you are having. As far as I know, if it's done properly, you should be able to do exactly what you're asking through the Script Extension. It's a matter, I believe, of defining properties on your RunTime object along with functions.
So, for example, you would have a method for "OpenExcelSheet" which would do exactly that, but it would then assign the Workbook object to a property within your run time object. You would then have a "Get" method for your property to get the sheet object that you opened.
If you would care to zip up your code, I'd be happy to help you with this.