The REC file only contains the raw image data, it does not contain any information regarding spatial orientation or dimension. You can deceive dcm2niix by providing any file of the appropriate size and giving it the .REC extension. You could even create a file of zeros, just specify the correct count in bytes...

  dd if=/dev/zero of=binary.dat bs=1c count=1


Alternatively, you could adapt the Matlab scripts of dicm2nii to do this for you:
  https://github.com/xiangruili/dicm2nii