Stefan, I would suggest using customVariables for this.
Just make sure that you put this code before calling _trackPageview.
_gaq.push(['_setCustomVar',
1, // This custom var is set to slot #1.
'VisitorID', // Name of your CustomVar that will show up in Reports
'123456789', // ID of your visitors, you need to set this.
1 // Set the scope to visitor-level.
]);
The tricky part could be using actually getting the visitor ID on consistent basis. One of the ways to go is to actually use the same Visitor ID as Google Analytics stores in _utma cookie (details in this article).
The complete code to get it done this way would be following:
var a = uGC(document.cookie, '__utma=', ';');
var id = a.split(".")
_gaq.push(['_setCustomVar', 1, 'VisitorID', id[1], 1]);
After that, you can build custom report with the VisitorID as the main dimension and select any metrics you would like to see. Just make you use the correct and logical combination of dimensions/metrics to get numbers that actually make sense (see this brilliant article by Avinash Kaushik).