I’ll explain it again: Webtop IS a multitenant solution since version 4, and still is version 5.
To work as a multitenant solution, you need a multitenant ldap backend, a multitenant imap server and a multitenant smtp server.
If you can manage to manually configure NS7 as a multidomain ldap, multidomain imap, multidomain smtp, then you can create webtop domains on these domains.
Any other trick will be a pain to manage.