casts for user defined types

Hi all,
    I am using inheritance of types in the following way on a ROLAP
schema to allow for extra dimensions to be used:

CREATE TABLE data (data_id  int4 SERIAL,
                    data_value double,
                    ... some dimensions);
CREATE TABLE extra_dimensions (xdimension varchar[],
                                xvalue varchar[])
              INHERITS FROM data;

Suppose I do
         INSERT INTO data (data_value) VALUES 1.1;

This gives me a row in the data table, but not in the extra_dimensions
table.  What do I do if I want to define an extra dimension for this row
at a later date?  It seems I should be able to do a CAST of some kind.

Thanks.

Mary Anderson