Pedro,
In order to mesh a volume with hexahedral elements you need to be able to either map or sweep mesh that volume. Typically you have to decompose the volume into smaller, simpler volumes and sweep (or extrude) a quad mesh into a hexahedral mesh. Volumes which are then too complex can be meshed with tetrahedral elements. Have you read the Ansys meshing documentation? It's a very good place to start for this.
To my knowledge there are no "free" purely hexahedral meshers in existence. Even the most sophisticated hexa meshing systems such as ICEM require some skilled user input.
Good luck,
-Brian