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