Quoting this backtrack faq
Why cant I just add the Backtrack repositories to my Ubuntu install or the Ubuntu repositories to my Backtrack install ?
We highly recommend against this action because Backtrack tools are built with many custom features, libraries and kernel. We have no way of knowing how they will perform on a non Backtrack distribution, plus you will very quickly break your install.
Also if you chose to add the ubuntu repositories to your Backtrack install, you will most certainly break your entire Backtrack install very quickly.
We do a lot of testing to ensure that all packages in our repo will work together without causing problems.
I think, Backtrack gone through some very critical changes, so Ubuntu + security tools may not be equal a Backtrack. But, you may have a distro very close to it.