Welcome to the Yin-side!
You are free to extend the beginner’s sequence by holding the postures longer, as long as that feel appropriate. If you want to work other areas of the body, there are other flows in my book, the Complete Guide to Yin Yoga, that can give you that. Check out the flow for the hips
and even the flow for the upper body. There is a whole section on upper body yin in the book.
Yang poses can be added but only as a short counterpose. Which yang posture depends upon what pose you just did. Generally we either try to relax the whole body, or make gentle movements in the opposite direction of the previous pose: for example – if you did Butterfly, which is a flexion of the hips and spine, you could do a gentle backbend (extension) of the spine via Table top.
To add Yin Yoga to your Ashtanga practice, you could do Yin Yoga one day, then Ashtanga the next day. If you have time to do both, if you are doing your yoga early in the day, start with Yin Yoga and then do the Ashtanga. If you are doing it at night, start with Ashtanga and calm yourself down with the Yin practice.