The only way to do this would to be ... 1. To be running Android "Marshmallow" on that phone (which is not what it comes with, so it would have to have been upgraded to it) and 2. The manufacturer would have to not restricted this option on that phone. They do that sometimes.
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...