Going from 7 to 8 speed opens a whole can of worms. Unfortunately everything has to match & fit for it to work. You will need special measuring equipment & tools. It's probably not worth the effort. You're much better off just buying another bike. But, if you're really interested in becoming a tinkerer. Start checking out the local for sale ads. Set price filters. Sometimes you can pick up a good condition 8, 9, or even 10 speed used bike or "parts bike" for anywhere between $75 to $150.