Hi Luca ,
You can download the mendix version 8.18.99 and create a sample project. In that you can download the dropbox connector and convert the project in to your version(9.15).
Now in that sample project export the dropbox module and import in your application.
I hope it helps.
There is nothing stopping you from downloading any module of version 8.18 into an app that is on version 9.15.
And if a module's version is too low, then you have the option to import that module into a new app that is also on that low version, upgrade that app, export the module and import your module-export into your app. But that seems not neccessary in this case.