Here is a code snippet that will allow to install suPHP:
Code:
cd /usr/local/directadmin/custombuild
./build update && ./build clean
for i in 1 2 3 4; do ./build set php${i}_mode suphp; done;
./build php
./build suphp
./build apache
./build rewrite_confs
But as it has been already mentioned you'd better use fastcgi or PHP-FPM instead of suPHP.