Well if it is running the original Android version, you won't be able to. However, if it has been, or can be, upgraded to "Marshmallow", then you may get it to work. The other factor is if the manufacturer has restricted that ability or not. Some do.
The only way to know for sure is to try. First, see if you can upgrade to "Marshmallow". If you can, or have, great, follow the instructions below to see if it will accept the change. If you can't update it to "Marshmallow", then your best bet to free up space is to move photos, music and videos to the card as apps won't move.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
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.