If you currently have a hosting plan which was setup before the Multi-domain feature was offered contact the Helpdesk for a free upgrade.
The “Add Domain” and “Resource Management” features will be added to your account within 24 hours thus enabling Multi-domain features..
It is not possible to merge existing separate hosting accounts into a Multi-domain hosting plan. However, at the time your accounts are up for renewal, if you find they are not using many resources you could allow your package to expire and add then add domain to your Multi-domain account.
If your hosting plan is currently on our Ensim server range, then you might want to consider upgrading to our Plesk server range – the upgrade is free, see Upgrading to Plesk from Ensim FAQ entry.