ou should write DreamHost support about this, as they may allow you to transfer that domain (hosting and registration!) to a "free" account for non-profits.
I say "may" because, generally speaking (and without intervention by a DreamHost "human") the DreamHost system will not let you transfer a domain that has ever been hosted on a DreaHost account to another "plan" that takes advantage of a promo code or pays a referral.
They do, however, strongly supports organizations that can prove they are a non-profit ( 501(c)(3) status required), and they may well help you with this.
If they are willing to set this up for your organization, it's a big win all around for the non-profit, as the program also provides a "free" domain name registration for the life of the account.