Using COPY for bulk upload in a table with sequence field

Hi:

Can I use COPY command to upload a flat text file into a PG table where
the PK  column (item_id) is Serial type.
The table is an existing table with data and I want to
insert new rows from the flat file. The sequence already defined for
this column.

If I do not have the item_id data in my flat file, is it going to get the
id from nextval from the sequence?

Any help is appreciated.

Gautam