Passing additonal userid to piwik

Is there some way to pass a userid from the source website to piwik as part of its unique user tracking ?? I’m thinking of tracking users and /filtering out employees… that have logged into Joomla, etc… So that I can track them across multiple sites

Check out piwik.org/docs/javascript-tracking#toc-other-functions

(after multiple edits) whoever setup url parsing on this forum is retarted