Can I add mint flavoring to ready made royal icing?
Yes you can in small quantities. My mum used to do a lot of cake decorating, and still does bits with my kids.
Adding small amounts of liquid such as flavoring/colouring is perfectly fine. It is important to knead it in properly. You can use a small quantity of icing sugar to prevent it sticking, but the main thing is to keep handling to a minimum where possible as the heat in your hands is more likely to make it sticky.

In terms of application, we typically use wooden tooth picks and dot the flavoring/coloring in, then repeat as necessary.