I don't think it's possible to do what you want (maybe somebody knows how to do this).
What I did, and suggest you to do is the following:
1. Install Sarge with 2.6.8 kernel. Just install the base system.
2. Once done that change from Xfree86 to Xorg.
3. Change your sources.list to point it to sid repositories.
4. Upgrade the base system (the core packages) that includes the kernel.
5. Upgrade the rest of the system
You can also do some search, In the meantime I'll search for the link to the how-to change from Xfree86 to Xorg.
I found it http://www.debian-administration.org/articles/185