Go to: Tools > "Auto Import options", add your directory to the auto-import list and ensure that 'run auto import in the background' (under 'options') is selected.
Create smart list:
[media type]=audio [filename (path)]="enter directory path here, keep the quotes."
Now, any files you add to that directory will be imported into your library automatically, and will then show up in your smart list.
You cannot manually delete a file from a smart list (unless you remove it from the library or remove it from disk), but, if you move files out of your directory, they will drop out of the smart list you created above. If you want to keep the files in your library, then use either the 'rename move & copy' files tool or drag and drop within MC "My Computer" branch of the tree. MC will then move the files for you and keep all the track and tag info connected. As the files will then no longer satisfy the filename path rule of your smartlist, they will not feature in it next time you view it.
-marko.