Well it is going to depend on two things. One what version of Android your phone is running. You will need to be already running, or able to upgrade to, Android "Marshmallow". and Two your phones manufacturer will have to not restricted this option on the device. And yes they actually do that with some devices.
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...