You can make a $1 pledge now, and when the Pledge Manager goes live after the campaign has ended, you can add more funds to get a full pledge and any add-ons you'd like.
However, you won't be able to get ONLY the add-ons. You'll need to get a basic pledge in order to enter the system.
Yes, if you double your basic pledge you'll receive an extra base game and an extra copy of all unlocked stretch goals. If you have an Early Bird pledge, extra pledges also cost $60.
Don't see the answer to your question? Ask the project creator directly.