Can someone be on my best friends list if I don't snap chat them back?
No, no one will appear on your Best Friends list if you don't chat with them. The more you chat with someone the longer they will appear in that list. There is no other way or possibility to appear on the Best Friends unless you actually chat with them.
There is also a support page on the Snapchat page where you can see more details about Best Friends: https://support.snapchat.com/en-US/article/bestfriends