Well, it will depend on two things. 1. You will have to be running, or able to upgrade to, Android "Marshmallow". 2. The manufacturer will have to not have restricted this option on that device. Which, sadly, does happen. Often. If you have, or can upgrade to, "Marshmallow", then try the following instructions.
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...