Add column

Hi, there is a feature that I would be glad if it was supported by
PostgreSQL:

I want to add a new column (attribute) to a pre-existed table, but instead
of filling the values of this column with a default value (e.g. null) I
would like to copy them from a file stored in my filesystem.

I didn't find any example of mixing "copy" and "alter table ... add
column" in the Documentation, however I send this mail to check it out.

Thanks.