What That Wouldn't Do
You can add Debian's software sources in Ubuntu the same way you add them in Debian. This will not do what you seem to be saying you want, though. You should almost certainly not do this.
This will not give you the same software that will appear in Ubuntu 13.04 in the future, because Ubuntu's packages are specially built for Ubuntu, incorporate some changes compared to the Debian versions, and because your package versions would almost always be ahead of the ones in Ubuntu (especially on Ubuntu 13.04's release date, which would take place well after a freeze on the introduction of new features).
Ubuntu's packages are not identical to the packages in Debian, and you cannot get the same exact packages through an official Debian software source that you get through an official Ubuntu software source.
What That Might Do
If you added Debian's experimental software sources to your Ubuntu system, they would almost always have higher-versioned packages than provided by the Ubuntu software sources (since most Ubuntu packages come from Debian). Consequently all the software in your system would be upgraded to the Debian experimental versions. This upgrade process would very likely fail, leaving your system or the package manager in a non-functional state.
However, if it succeeded, the result would be a Debian experimental system that happens to have some Ubuntu repositories enabled (but providing almost no software).
You mentioned that you don't need "Unity and stuff." By "and stuff" I presume you mean other packages present in Ubuntu but not Debian. Since you don't need any Ubuntu packages and you're interested in using a system where all the packages are provided by Debian's official software sources, it sounds like what you really want to do is use Debian.
Debian, like Ubuntu, is an excellent operating system. Go for it!
But please don't try to "upgrade" an Ubuntu system to Debian; you should install Debian in an officially supported way. If you want to use experimental packages, you can get plenty of excitement that way, and find and report bugs while you're at it. In contrast, there's not much value to making a broken Debian system out of an Ubuntu system.
To Stay on the Bleeding Edge of Ubuntu
If you want to use the latest development packages for Ubuntu 13.04 before it comes out, then instead of using Debian's experimental software sources, you should test 13.04.
New, experimental installation ISO's for Ubuntu come out every day and can be downloaded here. Once installed your alpha Raring Ringtail system will get lots of updates and plenty of stuff won't work right. It might even not work at all, or stop working at some point. (That's what it's like to use the most experimental software available.)
It's fun, though I don't recommend this for a system you rely on, even casually.
If you want you can upgrade an Ubuntu 12.10 Quantal Quetzal system to alpha Raring:
sudo apt-get update
sudo do-release-upgrade -d
If alpha testing interests you, check out these resources: