A secondary container assembly includes a skirt configured to securely and removably engage the rim of a beverage can. The secondary container includes a secondary compartment holding a secondary liquid. A primary flow channel extends through the secondary container to provide a fluid flow path from the open beverage can to an outlet of the secondary container. A plurality of mixing apertures provide fluid communication ports between the secondary compartment and the primary flow channel, allowing the secondary liquid to mix with beverage from the can in the primary flow channel. A vent tube prevents a vacuum from forming that would otherwise impede flow of the secondary liquid into the primary flow channel. A removable seal covers the outlet and mixing ports. A threaded lid holds the seal in place.