I am writing a story that is planned to be less than 10 000 words but it is in present tense and 1st person but there is an essential scene that needs to be in 3rd person, with it being a short story I can't make another chapter so does anyone have any ideas about how this could happen? any replies would be appreciated
Just write it in third person. If it really does need to be written in third person, readers will go with it. If it doesn't work just cut it, because no scene needs to be in a story at all.
Or do add chapters. Short stories with chapters (or something like it) aren't uncommon. Look at Jekyll and Hyde, as the most well-known example, or at some works of HP Lovecraft or Arthur Machen, or The Hunting of the Snark (by Lewis Carroll) (which isn't really a short story, but contains less than 10 000 words, I think). Most importantly, as long as it doesn't break the flow of the story, I don't see reasons why not to add chapters.