You would need someone to precompile source of all packages and get .debs ready for you (the Debian binary packages for Linux kernels won't work for FreeBSD kernels), and recompiling/porting is not always trivial. Ubuntu doesn't have those precompiled binaries, so I think the simple answer is "no -- at least not without a team of very competent developers".
You can, however, try to use Debian GNU/kFreeBSD and install the same packages that you'd have in an Ubuntu system. However, that distribution is not yet in great shape.
The same is true for other kernels -- there is Debian GNU/Hurd, but there is no precompiled Ubuntu packages for the Hurd, and several Debian packages are not available for Debian GNU/Hurd.