If someone is sufficiently ill on Yom Kippur to be halachically allowed to eat, would there be any value in him fasting for a whole day after he gets well? I'm sure it would be voluntary, but is it ever encouraged? Anything from the Sources?
In general, if one cannot fast Yom Kippur and is directed not to fast by his/her LOR then it is likely to be considered a case of Pikuach Nefesh and it is even considered a mitzva to break one's fast to preserve their life!
I dont have any sources off hand, but it is common knowledge in mainstream Judaism that when once is patur (exempt) from something such as fasting, they do not need to "make it up" as their obligation to fast has been discharged.