Well, it is going to depend on the version of Android the phone is running. If you are running the version with Lollipop, then it can't be done. However, if you are running the version using Android "Marshmallow" they you 'may' be able to do it. It depends on if Samsung has restricted it or not.
If you are running "Marshmallow" or can upgrade to it, then you should be able to try and make the change using the information below. NOTE: Make sure you are starting with a blank SD card. Why? Because if you are able to do as stated below, then it will format and encrypt the card, so anything on it would be gone.
How to turn external SD card into internal storage on "Marshmallow".
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.
When I clicked storage, it brought me to internal storage and didnt give me the option to select SD card