Optimaly this is all done via a common iCloud container… However the Prime31 iCloud plugin I use doesnt support common containers 
(does anyone know a plugin that does?)
…but isnt there a way for the paid app version to look for the localy stored cloudfile from the free version? This is done only once (the first time a player runs the payed version).
Is it necessary to divide into two separate versions? You could implement IAP in your free game and unlock full/paid content through that. Having your app scanning your device for files that belong to other app sounds like not the best plan at all…
No, i don’t think this is possible. I don’t have much experience with iOS (i only have Android) and never used iCloud, but all mobile platforms restrict access to app resources to the app itself to prevent other apps from tempering with data that doesn’t belong to them.
One solution would be to have some sort of “export” button in the free app which will store the relevant data in a single file in the user space of the device, and have the paid version consume that file on load. Keep in mind that all files in the user space can be edited / changed by the user. So you might want to use some light form of encryption to prevent easy cheating (depending on how sensitive the data actually is).
Another solution would be to use an external storage (webserver) to exchange the data. This however is a bit tricky since you need a way to uniquely identify a user device on your server.
Others suggest to use custom URI schemes to send information from one app to another.
Also have a look at the official iOS documentation. Most solutions would require some sort of plugin / native communication.