Not running Android "Lollipop". You would need to be running at least "Marshmallow". Now if you can get the phone to allow you to upgrade to "Marshmallow" then it will just depend on if the manufacturer has blocked the option or not.
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 &...