How To Use External SD Card As Internal Memory. Steps for Marshmallow & above are in the descripton
Download this app: http://q.gs/Dvls8 You can use your external SD card as internal memory in your android device. If you can't find the file "vold.fstab", your device cannot support this mod. Please backup your device before applying this mod.
Its very simple by modifying the code in vold.fstab file which is present in ext folder of Android's system folder.
This file so called vold.fstab has mounting information for the device to mount the SD cards so change the code to swap the SD cards so that ext_card will be mounted as internal storage and internal storage will be mounted as ext_card.
Note: This will work on any Android phone with internal and external SD cards.
Here is the procedure with an example I have done on Xperia J's vold.fstab file.
1. First of all your phone must be ROOTED, If its not then STOP here and get the root first.
2. Download Root Explorer from here: http://q.gs/Dvls8
3. Navigate to system/etc folder mount it as rw(read/write).
4. There you will find vold.fstab file, open it in text editor.
5. Replace sdcard0 with sdcard1 and sdcard1 with sdcard0 in those two lines.
6. Save the changes
7. Reboot the system.
8. If you want to restore changes, revert the changes done and reboot.
This feature is now native in Android 6.0, just follow the below steps to turn External SD card into Internal storage on Marshmallow
1. Go to “Settings” then select “Storage”.
2. Select SD Card and then tap the menu button on top-right corner or your native menu button and select “Settings” from there.
3. Select “Format as Internal”
4. Select “Erase & Format”.
5. Wait for some time as your SD card will be formatted.
6. Reboot the device.
All the best.