In practice, this would require modifying the software.

RawTherapee and darktable are open-source, so you can study and change their code to add or replace a demosaicing method. There generally isn’t a built-in way to “drop in” an external demosaicing algorithm and then resume the standard pipeline as if it were a normal option.

Also, demosaicing happens very early in RAW processing. Most later image-processing steps expect properly reconstructed color data, so stopping the pipeline before demosaicing and trying to do much work on the RAW mosaic itself usually isn’t very useful and can cause color problems.

So the practical path is:

• modify the source code to integrate your algorithm, or
• demosaic externally and then continue editing as a non-RAW image, if that fits your workflow.

If your goal is to test a new demosaicer inside a RAW workflow, adding it directly to RawTherapee or darktable’s source is the most appropriate approach.