You could edit the championship and choose a new weather preset for each race?
Actually, I got it working without changing anything! I found this pdf by Peter Boese himself (attached)
It explains how to get Pure working in championships. So once you've done what is shows in the document, you can leave your "Sol" weather presets as is and Pure will use that as the weather for the track, pretty neat!
Having that said though, rain tyres will still only be available on tracks that have the RainFX added of course.