Best answer by chicks
For some reason, Sonos has never added that capability to Tablet versions of their apps, an odd omission. You need to do it via a PC.
However, there aren’t too many stations that aren’t available via one of the many tuner UIs in the Sonos app, like TuneIn, Radio.com, Radionomy, etc. or, you can use AirPlay to one of the newer Sonos speakers.