Well for moving videos you shouldn't need to root the phone. However the phone itself may not have the ability to move the files without adding an app that can. The fastest way would be to connect the phone (while the SD card is still in it) to a computer (any computer) via USB cord. Then move all the videos over to a folder on the computer. Once they are on the computer, you just copy them over to the SD card (while it is still in the phone) via they same USB connection.
For future videos, if they are recorded on the phone, then you would need to set up the app that you use for it to save to the card rather than the phone. And if it is videos you are downloading, you would need to set up the app you are downloading through to save to...