This should be no problem.
The first step is for your wife to de-register the Kindle from her account. She can do this from the Kindle itself, or from the Manage Your Kindle page on the Amazon site.
Once she's done that, you can register the Kindle to your own account. Again, do that either from the device or from Manage Your Kindle.
If the device has never been registered to anyone, just go ahead and register it to yourself. Have you tried doing that? Have you come up against a problem?
Note added later:
I've just checked the Manage Your Kindle page, and I can no longer see how to register your device on that page. Maybe the system has changed since I last did it. But you can definitely register and de-register from the device itself. For step-by-step instructions, see
http://www.amazon.com/gp/help/custom...=200505460#reg
Mike