Well it is going to depend on which version of Android you are running. If you are running the older installed versions, then no. If you have "Marshmallow" then maybe. I say only maybe because a few manufacturers have decided to restrict this option.
If, you are running Android "Marshmallow" then you will need a blank SD card to start with. Why blank? Because when you set the card to be "Internal" rather than "External" storage, the card will be wiped, formatted and encoded. So anything on it would be gone. For good.
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 tablet.
NOTE: If you don't reboot the tablet, many things may not work correctly, so make sure you do.