These features do not exist in dcm2niix. If you are using Linux or MacOS, I would suggest you use fslmerge, fslsplit, fslroi. You are using Windows, you could use an old copy of dcm2nii for these functions (its also worth keeping dcm2nii around if you are dealing with archival datasets that predate DICOM, e.g. proprietary GE, Siemens formats). I would not use dcm2nii for modern DICOM images - the vendors are constantly updating their interpretation of the DICOM standard and so there is an arms race to support the latest features. My development efforts have shifted from dcm2nii to dcm2niix.