Yes, you can host additional web sites in your VDH account without paying for the add-on package, however, you will not receive any of the additional features and benefits that come with the add-on package.

You will be responsible for detecting the host header domain name (Request.ServerVariables) in your application name and redirecting your users (Response.Redirect or Server.Transfer) to the appropriate default home page.

The VDH add-on package provides the following added benefits:

1) a separate virtual server (IIS Canonical) for your website.
2) application is run in its own dedicated server process. (non add-on shares application space)
3) uses IIS host header redirection. (or a dedicated IP if the $14.95 package is purchased)
4) unique global.asa/asx, application and session variables.
5) unique FTP account.
6) unique virtual mail server (sites without add-on package use mail alias)
7) unique log files and statistics
8) separate SQL server database (disk space allocation comes from your 2.5GB of space)