Hello,
go to se21, put package and display..go to properties tab. go to software component and press F4. you can see that HOME is for customer development. when you create a custom package it assigned to HOME(software component). Standard package is created by SAP....so its never assigned to HOME. so copy is not possible........mismatch of software component is the main logic.......so its not possible...
Hope your problem will be solved.......you understand my point of view.....
Thanks
Sabyasachi