Well is is going to depend on which version of Android the phone is running and also if LG has restricted that option on that phone. Some companies have on some devices. Why? Can't say.
If your phone is running anything other than "Marshmallow" then you won't be able to do it. If you are running "Marshmallow" they try the instructions below. If the option(s) listed aren't in your phone (and you are running "Marshmallow") then LG has restricted it on that phone.
How to turn external SD card into internal storage on "Marshmallow".
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.