I've just started recently using model builder so this may be the most inefficient way to go about it. What about a combination of the spatial summary tool and the add join tool? so you run the spatial summary tool. It creates an output table, you then join that table back to your original table. Or maybe an Append (data management) tool instead of a join. Just append you summary output back to your desired target dataset.
Hope this helps and if not, eliminates some possible answers