When you load the data model for you new database, set the acs_objects sequence to start higher than the last number you used in the old database.

Then when you import your objects, specify the object_id in the call to acs_object.new.

The system level objects will get created with different ids but that is not an issue, just the visible objects, forums postings, news items etc.

You don't have to grab an id from the sequence as long as the integer you use is unique.