Yes, it is possible to specify a different directory to use for the DocumentRoot. The DH panel will label it "Web Directory"
When adding a fully-hosted domain, after choosing the serve you are adding it to, you get a form page with these options:
Where username is a selection box, and domain where you can type in a different directory name or path to use as DocumentRoot.
So basically you just replace 'domain' with 'public_html' in the appriopate field above, and you get
/home/username/public_html instead of /home/username/domain
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