In this new post to his site, Cal Evans shares a handy tip for those using non-Composer libraries in a Composer-friendly project - using classmaps to bridge the gaps.
A problem I ran into when starting this project is that the official MailChimp API wrapper for PHP is NOT a Composer package. Thankfully, the wizards behind Composer have thought this through. To facilitate using non-Composer packages in composer projects, all I had to do is add one line to my "autoload" section of my project.
Using this "autoload" section, you can get Composer to add the path as a namespace to the class mapping. This lets it load them up in the same way i would any other PSR-0 formatted package. This will even work if you have libraries that aren't PSR-0 as it finds all of the files and pulls them into the map automatically.
Link: http://blog.calevans.com/2013/07/21/using-3rd-party-libraries-in-composer-projects