In a post to his site Tomas Votruba provides a guide "for dummies" to using local packages along side your remote packages via Composer.

I wrote about pros and cons of local packages before. After year of using this in practice and mentorings I polished this approach to even simpler version that is easy to start with.

[...] There is no need to use Github, love open-source, understand package design or understand composer beyond PSR-4. No symlink issues, no forgotten composer update. Anyone can start using this!

The "dummy" packages he creates are essentially a simulation of a Composer package just in a different location (under a "packages" directory). This can then be autoloaded via the same Composer configuration you already use without too much trouble.