I've been searching for a way to do this. I use a slightly modified Colemak keyboard layout and I can't figure out how to add it as a supported input method in the new language selection in Windows 8. The installer that the Colemak guys distribute sort of works; you can choose the layout after installing it, but it forces the EN-us language, and can't be chosen for other languages.
If anyone knows how to do this properly, I'm sure the Colemak guys would appreciate hearing about it, and it would let me make the modifications I want to.
I don't know if this is the exact issue but there is a problem with MSKLC-generated layouts in Win8 (DP&CP).