Sorry it took so long to reply, but it is possible to extract one portal out of a shared install.   I would hope that anyone hosting sites on a shared dnn install would be willing to separate portals on demand for customers, otherwise they should not use shared installs.  

In any case, there may be easier ways to handle this, but I make a complete back up of the system files and database, and setup the copy of the environment on a development server.   This also involves some adjusting of alias names on one portal for allowing access under a test domain, but once logged in as host in the development environment, on the portal to be kept, you can delete all other portals on the install.   

If premium modules are involved, it may be necessary to purchase new licenses for the new separate install, or uninstall the modules if not being used by the remaining sites.  In some cases, I have had to remove file folders for old portals, and I will always review database tables to make sure all is clean, but this is usually minor clean up effort.  Once completed, a new backup  of the database and system files can be made for install on the new location.