Hopefully, there is an app that does facilitate it, its called Mesej Je. It may seem weird after seeing its name but it does work. It lets you send a message on Whatsapp without storing the number as a contact.
See How Mesej Je Works:
All you have to do is, open the app enter the phone number with country code and click on the “Open Whatsapp” button. You will then be transferred to the Whatsapp chat window, where you can write a message to that number.
Usage of the app is simple all you need is a phone number of the person to write a message in Whatsapp.