Generating (or retrieving) visitor profile ID? (no replies)

I've been reading up on the API for the new Visitor Profile feature in Piwik 2, but I'm a little confused about how the IDs work. I'm assuming that a unique cookie is generated for a visitor if one doesn't exist, which is then tracked through their hits.. but is it possible that can I generate the visitor ID's myself and pass that ID to the tracking code, rather than Piwik generating the visitor ID, so that I can link the ID to my own member database?

The Live.getVisitorProfile API takes a visitorId but there's no indication on how one should actually get that ID. Any advice would be appreciated.

Thanks!