If you don't want to wait for them to add this version you could probably try this:
Code:
cd /usr/local/directadmin/custombuild
echo "mysql5.6:5.6.37:" >> custom_versions.txt
./build mysql
You will probably need to manually upload corresponding mysql files from https://dev.mysql.com/downloads/mysql/ into /usr/local/directadmin/custombuild/mysql/