If you want to convert a NIfTI image to a generic DICOM (with the 'OT' other modality) you could try a tool like
  https://github.com/biolab-unige/nifti2dicom
If you want to save a NIfTI as a 'MR' modality DICOM, it will be tricky as there are required (type 1) DICOM elements that are not stored in the NIfTI. You will also want to ensure that proper unique IDs are created.

Different manufacturers interpret DICOM differently, and it is a very complex standard. It is easy to create DICOMs that are supported by many tools, but hard to ensure it will be supported by all DICOM conformant tools.