What can I mix with paint to avoid mold?
Depending on the paint you bought, you may be able to add lime which is a mold inhibitor due to its pH.

If you can still return the paint, making your own milk paint (aka casein paint) at home is an option. Milk paint has the property of being a natural mold inhibitor if done correctly. Numerous recipes (and youtube videos) can be found online with a quick search depending on your application.