Right-grading RH => Debian
You can install multiple package managers on a single system, but it's not recommended.
As Wade suggested, I'd converted my RH system to Debian under a chroot shell. I'd rebooted to start off the installation, on a partition cleared off for the purpose. Once the base system was installed, I booted back to RH, and did:
$ cd /debian-root
$ chroot . ./bin/bash
...which gave me a root shell under the Debian system. Install and configure additional packages. When things were sufficiently advanced, test out booting Debian. When you're happy, boot Debian and migrate your /home and /usr/local directories to the Debian side, and reclaim RH system directories.