Well, it will depend on what version of Android you are running. If the phone is using say "Jellybean" or anything prior to "Marshmallow", then no you can't download apps to the SD card directly. However, if you are running "Marshmallow", and the manufacture hasn't restricted this option on the device (sadly they do that sometimes), then the following should help you.
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.