how to append records from dump to existing database?

Hi,

I would like to know how I could append a database dump to an existing
database without clearing it.

Example: I got a database 'db' with a tables 't1' including variables
'var1' and 'var2', and table 't2' with variables 'var3' and 'var4'.
Earlier this year, I stored records 1 to 100 in this database and dumped
the whole database to a file 'dump1'. Then I cleared the tables and began
collecting new records 101 - 200. All variables are the same, no ownership
has changed, etc.

Now I would like to append the records 1-100 from dump1 to the existing
database. How could I use pg_dump or another command to append records
1-100 to the existing database? Please note that I do not have enough space
to first recreate the database from the dump and then merge it with the
newer one.

More generally, knowing that I will have to do this periodically, how can I
best dump current records to ease later merging/appending?

Thanks!!

Ana