Serial field

Hello to all

Unable to find an answer by searching the list archive, I ask my question
here:
I want to migrate a database to PostgreSQL 6.5.2
I need an auto-increment field. I first tried the SERIAL type, but it
doesn't fit my needs.
In fact, I want to:
1/ keep the original INT value of my existing records from another server
(say, first row can be "1", second "3", next one "17",...)
2/ have an automatic incrementation of the last value for a new record. For
exemple, with the values above and a SERIAL field, the default value for a
new field would be "1" (first use of the sequence) instead of "18" (last
value+1)..
I hope my english is not too obscure !
Any advice will be welcome
Regards

--
François THOMAS