Actually, the IPUMS data extract system does this for you automatically. If you select December basic monthly samples, you’ll notice the Food Security Supplement questions are available under the Household variables tab. Other variables, found in the December basic monthly files relating to labor supply, are available under the “Core” group of variables under the Person variables tab. The IPUMS CPS system is built on providing access to the basic monthly samples. Some of these basic monthly samples have supplements (such as the December Food Security Supplement) included. Therefore, in your case, there is no need to perform any merges of data.