This is out of scope for MRIcroGL and dcm2niix. The NIfTI files created by dcm2niix reflect the series as acquired on the MRI scanner. ChatGPT can help you write a simple Python script to concatenate your NIfTI, bval and bvec files. However, be careful as head motion may have occurred between sereis, phase encoding polarity may have changed, shim may have changed, or other properties may vary across series.

Rob Smith's insightful comments seem relevant to this question

https://community.mrtrix.org/t/concatenate-and-average-dwi-data/4029/2