I found out, USB Wifi support is all but non-existent right now. The only USB-N stick with free drivers is no longer on the market. And only one older G-standard chipset available.
Wifi has always been a problem for free software users. USB Wifi cards are becoming less free. With the older 802.11G standard many USB wifi cards had free drivers and did not require non-free firmware. With 802.11N there is only one chipset on the market, from Atheros, which is completely free.
I will try my luck with a USB-Ethernet adapter next. Apparently, the TrendNet TU2 ET100 adapter should be supported by a kernel driver.
This one with an Asix chipset might also work