[MrsDavis3]

You can use circular needles for flat OR circular knitting.

How: Just cast on and knit without joining the ends of the first row (joining the ends of the row would make a circle and lead to knitting a tube.)

BUT I wonder: Are you asking if you can change the pattern to make a tubular piece, or are you asking if you can use the circulars to make a flat piece? You could do either, but of course the tubular scarf will be twice as thick, and half as wide unless you double the number of stitches in a row (it would be called a round if you knit circularly).