You should be able to build a device from a cell if C is _|_ to the AB plane, so that is not a problem for an hexagonal cell. If Device from Bulk is not working, I think in two scenarios: 1) the cell is not long enough in the C axis, which you can solve using Bulk Tools -> Repeat. 2) C is not along the z axis, which you can solve using Builders -> Surface (cleave).