Add a device
How Can I Add Another device to my account ?
Add a device in what way ? If you want to register it to your account then you should be able to do so via this page : https://supportprofile.apple.com/
If you mean so that it appears in the 'manage devices' section on your account then turning on automatic downloads on it or re-downloading past purchases on it might do it (though I'm not sure what associating a device achieves apart from stopping it being linked to a different account for 90 days) : http://support.apple.com/kb/HT4627