Maybe a little late, but for others to read:
We ran into this exact gap ourselves and ended up building the missing piece — and we've open-sourced it: hestia-to-da —
https://github.com/maendamedia/hestia-to-da
It converts a native HestiaCP user backup (v-backup-user output) into a native DirectAdmin backup that restores straight through Admin Backup/Transfer. The output follows the same conventions as DA's own cpanel_to_da.sh, so nothing custom is needed on the DA box and no panel access is needed on the source.
What carries over: web files, SSL certs, aliases (as pointers), mail accounts and complete mailboxes with original password hashes (both panels run Dovecot with the same Maildir++ layout, so folders and read flags survive), forwarders, MySQL/MariaDB databases and users with original password hashes, DNS zones (with optional IP rewrite), DKIM keys, cron jobs and FTP accounts.
Known gaps are documented in the README (PostgreSQL, Roundcube address books, autoresponder texts, and one manual DKIM copy step after restore).
We've used it for our own production migrations (HestiaCP 1.9.x → DirectAdmin 1.7x on OpenLiteSpeed).
MIT licensed — feedback and test reports with other version combinations are very welcome.