Combine subdomains into one domain
The Domain Access module was created for exactly the sort of use-case you're describing.


  The Domain Access project is a suite of modules that provide tools for
  running a group of affiliated sites from one Drupal installation and a
  single shared database. The module allows you to share users, content,
  and configurations across a group of sites such as:
  
  
  example.com 
  one.example.com 
  two.example.com 
  my.example.com
  thisexample.com &lt;-- can use any domain string 
  example.com:3000 &lt;-- treats non-standard ports as unique
  


It'll take a fair amount of work to convert 6 individual sites into a single Domain Access site, but if that's the workflow you're looking for, I don't think you'll find an easier or more complete existing solution.