adding columns with defaults is not implemented

Hello,
Using Postgres 7.4, I am trying to perform an "alter table temptable add column "myCol" serial"

It gives the following msg
ERROR: adding columns with defaults is not implemented

You cannot add a column that is serial in a table which already has data in postgres 7.

Is there a way I can create a serial column on a table which already has data? Or is the only solution upgrading to postgres 8 ?

Thanks