OK, so reviewing the basics, each spreadsheet package has a workbook part, and some number of worksheet parts.
Using java Syntax Highlighting
SpreadsheetMLPackage 
pkg = SpreadsheetMLPackage.
createPackage()
 Parsed in 0.011 seconds,  using 
GeSHi 1.0.8.4
automatically creates the workbook part:
Using java Syntax Highlighting
        public static SpreadsheetMLPackage createPackage
() throws InvalidFormatException 
{
                
                
                // Create a package
                SpreadsheetMLPackage xlsPack = new SpreadsheetMLPackage
();
                try {
                        
                        xlsPack.
wb = new WorkbookPart
();
                        xlsPack.
wb.
setJaxbElement(
                                        Context.
getsmlObjectFactory().
createWorkbook()
                        );
                        xlsPack.
addTargetPart(xlsPack.
wb);      
 Parsed in 0.012 seconds,  using 
GeSHi 1.0.8.4
 
  and you can access it from the pkg, with pkg.getWorkbookPart()
So to glue in what I posted yesterday, use pkg.getWorkbookPart().getContents().setDefinedNames(definednames);