Conversion from NIFTI to DICOM format is bit easy. You can just do what David, Lee and Ramesh said.
If you want to perform a conversion from DICOM to NIFTI, I found this post that may help you

https://www.researchgate.net/post/How_ca...

where someone posted a matlab code as example.

This also may help a bit, someone use the niftitodicom command from vtk-dicom package:

http://stackoverflow.com/questions/29966...

or, this one is also a good choice regarding your post:

https://sourceforge.net/projects/uw-over...

The final option may be to do it by yourself in Matlab, maybe using the readnifti (attached) and the dicomwrite commands.

bye.