I guess I don't understand what you're trying to achieve or what types of actions you are anticipating. You can absolutely add things to Playing Now without affecting playback. Perhaps it's the double click action that has you hung up? If that's the case, you can change the double click behavior in several ways. By default, as you said, double clicking replaces Playing Now, with the album, or song you click on. I find this behavior disruptive; luckily there are some good options. Look in:
Tools > Options > General > Behavior > Double Click
I have mine set to "add to playing now (play now). What happens is, when I double click a file, it starts playing, and is added to Playing Now immediately after what was just playing. It plays immediately, but it DOES NOT replace Playing Now at all. The entire list is still there. So you can build your list right inside of Playing Now and double clicking still immediately plays what you clicked on.
If that doesn't work for you, you can certainly just have a scratch list that you use only for adding to Playing Now. Keep Playing Now as is. Build your scratch list while Playing Now plays. Then whenever you're ready, just right click on your scratch list and choose "add to playing now". That will put it at the end of Playing Now, so it'll play when the other songs (that were already there) are done.
To me it seems like you're trying to work against the system. I'm trying to point you in ways that will help you work with the system and make your life easier. But perhaps I don't understand all of your goals for this system.
Brian.