On a phone using "KitKat" you cannot. You have to be using at least "Marshmallow" to change the SD card into "Internal" Storage. Now if you can somehow upgrade to "Marshmallow" then the ability to make the change will depend on if the manufacturer has restricted this option, even on that version of Android. It happens. If it will allow you to change it, here is how to set it up.
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 phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.