The ratings don't appear to match exactly.
Freecharge: 3.5v 1000mAh
Zaurus: 3.7v 1800mAh
I think if the freecharge were putting out more than the Z needs you could step it down with the plug-in module part of the freecharge. This is lower though. You'd be underpowering the Zaurus.
I'm no electrician, but I bet this would shorten the life of your device, or possibly have even worse results.
[edit]
I appear to be an enormous dunce.
I was looking at the battery not the charger!
The Zaurus power adapter's output is 5v 2A.
This is what confused me because there's no mention of output on the web site, just the capacity of the freecharge battery.
Just as a point of reference my cell phone's power adapter output is 5.2v 800mA