If I understand you correctly, you need to move one of your portals to a new DNN instance because you need it to have its own Host account?  I can only assume that you do not want this new host account to have access to the other portals, because otherwise you are looking at a lot of work.  I say this because you can indeed at another Host account using the User Accounts module in the Host Menu.

The below steps will help you get this done, but any modules that are installed that do not implement the iPortable interface will give you problems.  That being said, you should also know that I have heard very few success stories centered around the exporting and importing of entire portals.  This mostly due to the previous iPortable statement, and synchronization issues that were not planned for before the import process (e.g., required modules, skins, and files).

In order to move one of your portals to another DNN instance, you need to first create a portal template using your current installation.  Go to the Portals module in the Host Menu.  Click the link labeled "Export Portal Template". 

Choose the portal you want to export and be sure to check the checkbox for "Include Content".  Click the "Export Template" link and save it somewhere accessible to your web browser.

Create your new DNN installation using the same version as the original.  Be sure to include all modules and skins that your exported DNN site depends on.  Also include any files that it used in the Portals directory.

Transfer your portal template to the new site.  It is probably best to put the template in the root Portals directory.

Login as a host and go to the Portals page again.  Click "Add New Portal" link and the form should be self-explanatory.  Just be sure to select your portal template.

If we lived in a perfect world, this is the part where you would be able to see your site and everything is in the same place as it was.  But this nearly never the case. 

Another thing to note is that I do not remember if any of your users are included in the export template, but I am 99% sure that they are not.  You will need to do some SQL magic to get them transferred over.  However, there are DNN modules out there that will perform that heavy lifting for you.  Take a look at your favorite DNN merchant site.