First, your tablet would need to be running, or upgradable to, Android "Marshmallow". Second, your device would need to have not be restricted by the manufacturer. Which sadly does happen. If you are running, or can upgrade to, "Marshmallow" then the following is how to change the SD card to be "Internal" rather than "External" storage. If you can't use these instructions, and have "Marshmallow" then the manufacturer has restricted that device.
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 tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.