Is it possible to put a single field of views in panels? There is an existing views with a few fields output. I would like to put one of these fields in panels. Can I get it directly in Add Content of panels?

If it is impossible, what is the best way to put a single field in panels?
Yes, you can add a single field directly from panels using Add Content, for this you may need to add a context with your entity. 


use Arguments option to pass an entity id from url argument and load dynamically context based on that entity;
use Context option to manually specific a entity id to load its context;


You can check node view page as example, which has node context by default.