Catalysis of the generic reaction: epoxide + CO2 + NAD+ + electron donor = beta-keto acid + NADH + reduced electron donor; the electron donor may be NADPH or a dithiol. This reaction is the ring opening and carboxylation of an epoxide; for example: epoxypropane + CO2 + NAD+ + NADPH = acetoacetate + NADH + NADP+.