Honestly it's an edge case, because there are few loanwords that don't get involved in the declension system. For some reason the government made official guidelines that the system only applies to native words, but that didn't stick, and most loanwords do have declension forms. With "kiwi" being one of the exceptions of course.

Going back to the question though, I can't think of words I could add without changing the overall structure. As in, our translation of "homo homini lupus" is "człowiek człowiekowi wilkiem", and it's not like you could "just add something to make it full form". Well, you could say "człowiek człowiekowi jest wilkiem", with "jest" meaning "is", but when you say "kiwi kiwi jest kiwi", it still sounds like garbage. I guess the only way out of this would be to use something different, like "kiwi dla kiwi jest jak kiwi", which is "a kiwi to a kiwi is like a kiwi", but that's not what we want, because when we talk about people and wolves again, it becomes "człowiek dla człowieka jest jak wilk", and now it's clear that the cases have changed.

Giving an example of a loanword, the government's official position used to be (or even still is?) that "radio" has only one form, but if you ask me, "radio radiu radiem" sounds clear and natural.