Copying data from one table to another - how to specify fields?

Hello,

I'm using CentOS 5.5 with PostgreSQL 8.4.7 and
am migrating my site from phpBB 3 to Drupal 7.

I would like to copy these fields from one table:

  select user_id, username, user_email, user_regdate, user_lastvisit
from phpbb_users where user_id > 50;

into the other (already existing) table:

  select uid, name, mail, created, access from drupal_users;

I've read http://www.postgresql.org/docs/8.4/static/sql-selectinto.html
and few more docs but don't understand, how could I specify the
fields correspondence (i.e. phpbb_users.user_id -> drupal_users.uid)

Also the target table exists already, has some records already
and has more fields than specified in my select statement above.

Thank you
Alex