from_dict
You can manually add a dict of ndarrays (assumes dim order as chain,draw,*shape) for each group you want to populate.
This will return a new InferenceData object, which can then be merged with the original InferenceData (one from from_pymc).
You might want to try a few times how it works in Jupyter Notebook (html_repr makes it easy to inspect). <-- this is especially true when trying dims and coords.