Type in what you want in cell A1 (for example 11K01) click on the cell then in the bottom right corner of the cell there will be a small black square, click and drag down as many cells as you want and it will automatically do this in order. Works 100% as I've just checked it to make sure + I've got top grades in excel programming 2 months ago in college
Basically what you have to do is similar to what zoltan said, write 11K01 in cell A1 and select it then drag down but not from the center of the cell but from the bottom right corner