First, they're missing since WordNet only contains the open-class words as described in their page:
Q. Why is WordNet missing: of, an, the, and, about, above, because, etc.
A. WordNet only contains "open-class words": nouns, verbs, adjectives, and adverbs. Thus, excluded words include determiners, prepositions, pronouns, conjunctions, and particles.
And so other types of words which do not exist in WordNet are actually closed-class words, in the sense that they have fixed number of entries. Therefore, you can always hardcode them into your program.
You can find the list of pronouns, conjunctions, etc. on this website.