Adoptable storage (that means extending the internal storage with the SD card) is not available on Fairphone (Open) OS because that feature was introduced on Android 6.0 Marshmallow and Fairphone (Open) OS is based on Android 5.1 Lollipop. Work on a new, Marshmallow-based Fairphone (Open) OS is almost finished so the “adoptable storage” feature will probably be there in a few months.
The “write protection” on SD cards can easily be removed after switching on root in Fairphone Open OS.
The Fairphone 2 supports an external SD card reader (according to my tests), but destroyed an USB stick of mine. (So the external key file should be placed on a SD card, not an USB stick.)