Hi, Joanna-
NIFTIs are perfectly acceptable in the AFNI software.
Personally, my usual "go to" for converting DICOMs is dcm2niix, which Chris Rorden & Co. have kindly contributed into the AFNI distribution itself (under the name dcm2niix_afni, so that it doesn't conflict with any other native version of dcm2niix you might have on your system).
--pt