For Custombuild 2.0
cd /usr/local/directadmin/custombuild
./build update
vim options.conf

curl=yes
./build curl
./build php n

For Custombuild 1.2 (will get error)
[root@mail custombuild]# vim /usr/local/directadmin/conf/directadmin.conf
[root@mail custombuild]# vim options.conf
#Advanced Settings
autoconf=yes
automake=yes
libtool=yes
curl=yes
new_pcre=no
[root@mail custombuild]# ./build curl
You cannot install curl, because you do not have it set in options.conf file.

mv custombuild custombuild_bak
mv custombuild.tar.gz custombuild.tar.gz_bak
wget http://files.directadmin.com/services/custombuild/2.0/custombuild.tar.gz
tar xvzf custombuild.tar.gz
cd custombuild
./build
./build curl

[root@mail custombuild]# /usr/local/bin/curl -V
curl 7.61.0 (x86_64-pc-linux-gnu) libcurl/7.61.0 OpenSSL/1.0.1e zlib/1.2.3
Release-Date: 2018-07-11
Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IPv6 Largefile NTLM NTLM_WB SSL libz UnixSockets HTTPS-proxy
[root@mail custombuild]# /etc/init.d/httpd restart