Easily. Just go to the sandisk card drive letter and open it up in a window. Then find your folder with your music files and copy the songs you want. then go back to the sandisk card and paste them in.
Your sandisk drive letter should be able to be found under "my computer". That location used to be easy to find right on your desktop. Lately the Microsoft geniuses have been hiding it on the start menu, possibly behind another name you see when you click "start"