EDIT:
Previous method no longer works in RTM version. The only method I can think of is using Microsoft Keyboard Layout Creator. It's kind of 3rd party though it's provided by Microsoft.
Microsoft Keyboard Layout Creator
Create new keyboard layouts from scratch
Base a new layout on an existing one
Modify an existing keyboard layout and build a new layout from it
Multilingual input locales within edit control fields
Build keyboard layout DLLs for x86, x64, and IA64 platforms
Package the resulting keyboard layouts for subsequent delivery and installation
I think you might be able to do in the registry at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout though I'm not sure how as it's not as simple as the above method.