English is always adding new words. It's a bastardized language anyway created by Normans trying to pick up Saxon barmaids. However, there is a process to add them.Words have been "added" to the English language according to various dictionaries. "Ain't" isn't word, but there it is in most dictionaries. Same with illegal as a noun. It traditionally wasn't considered a word, so it's hard to really consider it to be one.
"New words are added to dictionaries by lexicographers, who are experts in language who constantly monitor and research the use of words in various sources. Words are considered for inclusion if they demonstrate widespread usage, have meaningful and consistent application, and are used across diverse media. "
IOW, ain't got used until it really was a word. It wouldn't surprise me if illegals did. It's not a kind world.