dcm2niix will generate bvec/bval files if they are specified in the DICOM images. It should automatically work for Philips, GE, Siemens, Canon and UIH images. I would make sure you are using the latest stable release (currently v1.0.20211006). 
  Typically if these files are not generated, it is because you are converting a derived DWI image (e.g. TRACE, ADC, MD, etc) rather than raw diffusion data. You need raw directional diffusion scans for bvec/bval files.