This is not a very good idea, and there is no to do this. Just create a functional index.
CREATE INDEX ON table ( ROUND( (ST_Area(geom)/1000)::numeric ,2 ) );
I would create the index on the unrounded version.
CREATE INDEX ON table ( ST_Area(geom) );
Then a query like this..
SELECT * ON table WHERE ST_Area(geom) > x;
Will use the index. Alternatively, you can create a MATERIALIZED VIEW. Generally, it's not a good idea to cache the result of a calculation on the table.