I would highly recommend against trying this, for several reasons:
1) and most important: you won't get support either for your Plesk installation or your DirectAdmin installation, because neither support group knows what happens when you install the other. You'll be entirely on your own with every problem you ever have.
2) Plesk has custom RPMs. So does DirectAdmin. (Same for deb files if you're using Debian).
3) Email simply will completely break because DirectAdmin and Plesk use completely different email programs, completely different email layouts, and a completely different method for handling virtual mailboxes, anti-spam and anti-virus.
4) Competing daemons will try to bind to the same ports and will fail to start.
5) Locations for many programs, daemons, and files are very different; for example, Plesk's path to site DocumentRoots are very different than DirectAdmin's, they have different names, different paths for cgi-bin, and a completely different method for managing virtual services for httpd.
6) Setup files are completely different.
Note: I offer support for DirectAdmin and I spent two years as a Plesk Gold Partner, and even I won't support DirectAdmin isntalled over Plesk. Good luck in finding someone who will.
Have fun.
Jeff