Heureka, it works! Surprisingly my Huawei E173 WCDMA 3G USB Wireless Modem Dongle Adapter SIM TF Card HSDPA EDGE GPRS works as it should on the chromebook. Just plug and play, no fiddling with drivers.
The chromebook will open a new menu with different settings. Nothing needs to be changed, but the ChromeOS seems to be ready for all 3G settings.
An hint in the message box tells me that the chrome will use the 3G connection if WiFi is offline.
Now the chromebook will be even more online.