You're right. What I meant was that, aside from the irregularly pluralized nouns, you can add "es" or "s". For example, the word "virus" has no plural in Latin, but we pluralize it to "viruses" -- a Latin word with an Anglo suffix.

Since "athlete's foot" is a mass noun, it was questionable whether it even had a plural. But, on page 61 of A Concise Grammar of Contemporary English by Quirk and Greenbaum, it says "Virtually all non-count nouns can be treated as count nouns when used in classificatory senses:
There are several French wines available (= kinds of wine)

If it's a count noun, it has a plural. I mistakenly thought that since it was a compound noun, the plural would be formed like "${noun}s" or "${noun}es". But according to this page, it appears that "athlete's foot" undergoes regular plural inflection and thus the rightmost word would become plural. According to this logic, "athlete's foot" does have a plural and that plural is "athlete's feet".
That, and googling "athelete's feet" returns about 80x the results of "athlete's foots" ;)