Conversion to SUPHP in directadmin is easy. You may please proceed with the following steps

1) Set the following custom build options

/build set php5_cgi yes
./build set php5_cli no

2)Now compile apache and php using the following commands

./build all d
./build rewrite_confs

3) Now need to fix the roundcube , squirrelmail and phpmyadmin permissions as follows,

./build roundcube
./build squirrelmail
./build phpmyadmin

4) Now need to fix the ownership and permission of files and dirs as follows,

# for i in `/bin/ls /usr/local/directadmin/data/users/` ; do chown -R $i.$i /home/$i/domains/* ; echo $i ;done 
# for i in `/bin/ls /usr/local/directadmin/data/users/` ; do find /home/$i/domains/ -type f -exec chmod 644 {} \; ; echo $i ;done
# for i in `/bin/ls /usr/local/directadmin/data/users/` ; do find /home/$i/domains/ -type d -exec chmod 755 {} \; ; echo $i ;done

This easy steps will help you to conversion your directadmin to suphp and fix the permission issues in your scripts.