You would need to move them over to the SD card. The fastest way would be to connect the tablet to a computer via USB cord. Move them all over to an empty folder on the computer and then, while still connected, change over to viewing the SD card rather than the devices "Internal" storage. You can then move or copy the files over to the card. Personally I would suggest just copying them so that you have backups on the computer.
Once you are done disconnect from the computer. You would also need to go into the settings for the app you are viewing them through and make sure it is set to view them from the card and then set what ever app you are using to download them to download them directly to the SD card (if it will allow for it).