You may use the willow branches (aravot) from a lulav for hoshanot, but you must add three more to the two already on the lulav.

To set up a lulav: hold with spine facing you, with three myrtle branches (hadassim) on the right and two willow branches (aravot) on the left.  It is customary to put the myrtle and willow branches into a holder made of palm leaves.
NOTE Before Sukkot begins, open any sealed plastic bags that contain the willow and myrtle branches, as you may not cut them open on the festival days (the first and second days outside of Eretz Yisrael).