Converting 3GPP2 Multimedia Files directly to MPEG-4 Videos (.3gpp2 to .mp4 conversion) is certainly possible and should be quite straightforward.
Use one of several available video editing programs or (online) conversion tools, such as Movavi Video Converter or Wondershare Video Converter, which allow users to import 3GPP2 videos and export them in MP4 format.
Open 3GPP2 file Open MP4 file