What version of QuantumATK are you using? In principle, you must be able to use the Nanowire plugin to build nanowires/nanorods in the Builder, at least for cubic cells.
Otherwise, one needs to creative and, e.g., cleave a crystal structure in the crystallographic direction (and perhaps choosing proper in-plane lattice vectors) that is aligned with the nanowire axis. One then has to shape a nanowire of interest by manually cutting off the nanowire edges in the Builder, which can be done by selecting and deleting certain groups of atoms to make vacuum padding around the nanowire/nanorod.
