Well, it would need to not be blocked by the manufacturer (the ability to make this change) and it would need to be running Android "Marshmallow" or able to upgrade to it. Try the following and see if it will work on the phone. If not, then it is probably blocked.
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...