Is there a way to update custom packages with composer?
You do this with requiring the Composer Merge Plugin in your Drupal project's composer.json and then define in which additional locations it should look for composer.json files to be merged.


  Composer Merge Plugin is intended to allow easier dependency management for applications which ship a composer.json file and expect some deployments to install additional Composer managed libraries. It does this by allowing the application's top level composer.json file to provide a list of optional additional configuration files. When Composer is run it will parse these files and merge their configuration settings into the base configuration. This combined configuration will then be used when downloading additional libraries and generating the autoloader.