The problem I see is that DirectAdmin supports both Linux and FreeBSD, and they're based on different kernels with different structures.
Since firewalls work at the kernel level they'd have to work with two different code-bases.
And of course they'd have to work without interfering with more sophisticated firewalls many of us create.
Wouldn't just using Webmin make more sense if you really don't know how to open and close ports yourself?
Jeff