Is it permissible to feed a poor person for a missed day of fasting that I am making up, knowing that I am [currently] fasting?

If you delay making up the obligatory days without a valid excuse until the start of another Ramadan, you are sinful, and you must make up the fasts along with an expiation of feeding one poor person for each day. However, if the delay was due to a valid excuse, then you are only required to make up the fasts. It is recommended that the feeding be concurrent with or after making up the fasts, but this is not a condition; it is permissible to give it whenever you are able, whether before, during, or after making up the fasts.