If you’re a time-traveling story… SURE! Outside of that, you’ll probably confuse the readers if it’s extensive. (Aside from short flashbacks, or a brief start near the climax along with “two years earlier...”) This confusion is the main reason I broke up my first story I posted here, I was trying to cram in backstory into chapters as well, and came to realize I basically had multiple stories instead. (Which, while I’ve yet to really write that first story, I’ve been busy on the others.)