Hi Kirk,
I am not sure if you have gotten around your issue yet, I am curious to know if you tried joining the tables based on the Hydrant Number Field. I believe you can then select features with a NULL Shape fields (possibly using a definition query on said table to constrain the listed features)and use field calculator on the SHAPE column to make (NULL)Shape = Shape from the populated table.
I am not sure if you can do this with the Shape field as you can with other fields. I know that the you must generate the join from the table you wish to edit.
Hope this helps!