I don't have very much time to explain (but a little more than I thought I had before), but...
Why not just import those files into MC?
It is silly to go through a bunch of work to prevent MC from seeing them, especially since you could use MC to manage them later if you ever need to. If the issue you're worried about is all of those files being "in your way" when you look at your media through MC, this is easily prevented.
MC's Views are basically all just filters. They come with some stock filters, but you can add your own. If you add a filter to the top level Audio view, it will "trickle down" to any views underneath it.
There are a bunch of ways you can filter out files automatically, so they don't show, and aren't in your way, but where MC still has them in the database (and then, if desired, you can make other views that do see them, and maybe only them, so you can deal with them if you need to). One simple option would be if they are all in a particular folder on your filesystem (M:\Music\iTunes_Music\x\x\x, for example), just add a filter to your top-level Audio view that says:
[Filename] does not contain M:\Music\iTunes_Music\
Then, nothing under Audio will ever show those files. They'll be in the database, but they'll be hidden from those views.
If they're spread around into different folders, there are lots of other strategies. If it was me, I'd make a Tag On Import rule that assigns any files that belong to iTunes the [Keyword] "iTunes". Then, I'd exclude those files from my main Audio views by adding this rule to the Audio view:
[Keywords] is not any iTunes
I do this with a few different "special keywords" in my views. I have two keywords in particular: "Exclude from Views" and "Test File".
Then, I have a separate top-level "Advanced" view where I can see ALL files, including those that are otherwise excluded. I use these for a variety of tasks. Exclude from Views is nice because I can easily apply it to any file or set of files that I want to "keep" (for whatever reason) but don't want to see. Often this is when I've just imported a giant pile of disorganized media that I don't feel like dealing with right away. I just select them all, set the Keyword, and they all vanish until I want to go looking for them when I have time.