Actually, you can.
You need to use adb. You can choose to adopt a whole SD card or just part of the SD card.
IMPORTANT: this procedure will wipe your SD card (backup!) and not all apps can be transferred to the adopted storage.
Assuming you already have adb installed (otherwise, use the Minimal ADB app on your PC: http://forum.xda-developers.com/showthread.php?t=2317790)
- Open command prompt/Minimal ADB on PC, then type: adb shell
- Once connected to phone, then type: sm list-disks
- Note down the ID of your SD card.
- To adopt the whole card, type: sm partition disk:ID private where ID is the ID of your SD card. Eg. sm partition disk:179:160 private
- To adopt part of the card, type: sm partition disk:ID mixed % (where ID is the ID of your SD card, and % is the amount of space to remain as an External SD card.) Eg. sm partition disk:179:160 mixed 50
- Open the storage settings. Your available storage amount will be messed up, but it will still work.
- Select and open the adopted SD card partition. Tap the three dots in the top right hand corner. Select “Format as internal”.
I recommend that you use the mixed adoptable storage for two reasons:
- It gives you flexibility
- More crucially, the Sony camera app refuses to recognise the internal or adopted storage – it will only work if you have a standard SD card partition. Curiously, other apps (eg. Snapchat) do not seem to be affected by this.
Source: Me. I’m running 6.0.1 on a Z3 with a 64GB SD card, 90:10 split (adopted:standard).