Swap Command and Alt keys on USB keyboard
Using the IORegistryExplorer to check the internal and USB keyboard settings I found another property of the provider, alt_handler_id, set for the internal keyboard. In the same way I added the property Swap command and alt for the IOHIDEventDriver as follows:
First, open the property list for editing (as system) using a text editor capable of authenticating. I just used vi:
sudo vi /System/Library/Extensions//IOHIDFamily.kext/Contents/PlugIns/IOHIDEventDriver.kext/Contents/Info.plist
... IOKitPersonalities Swap command and alt 1 ....
touch /System/Library/Extensions
in Terminal followed by a reboot, the USB keyboard's Alt and Command keys were changed.
Furthermore, with this kind of keyboard you may want to change the input method. For example, a keyboard layout can be created with Ukelele. For me, the layout offered here works perfectly. Just download it and copy to /Library/Keyboard Layouts.