I would like to create a lot of buckets via API but I need to have a default alias assigned to them. Is there any way to do it not manually?
You can use the DreamHost API to configure CNAME records in the control panel. You'll want to point the record to .objects.dreamhost.com
Here's the wiki article with an overview of our API - http://wiki.dreamhost.com/Application_programming_interface
(note that DreamCompute uses a separate API)