You can use Import section of Segmentations module. You can specify spacing, axis direction, and origin position by choose a reference volume (click on Representations / Binary labelmap / Update, then click the conversion path, and click “Set geometry from volume”; you may enter oversampling factor > 1 to increase resolution).
You can get voxels of a segment as a numpy array using slicer.util.arrayFromSegment. You can also export the entire segmentation to a labelmap volume and access its voxels using slicer.util.arrayFromVolume.