I'm not sure you can do it directly with the regular Java APIs, I imagine you would need some third party library. or I suppose you could find out the structure of the Excel data file and just try to recreate that. I think an easier solution would just to creat a CVS file. Excel can load that just fine.
Using a Java-COM Bridge like J-Integra for COM, you can easily access/modify MS Excel from Java client. You can find more information about it at http://j-integra.intrinsyc.com
J-Integra for COM's pure Java runtime talks to COM components using Distributed COM (DCOM) layered over Remote Procedure Calls (RPC). The RPCs are themselves layered on TCP/IP. Therefore, at the lowest level, J-Integra for COM uses standard Java networking classes.