I have my MP3s on an external FW HD. My MC libraries are maintained in %My documents%\mp3 catalogs which resolves to T:\My Work\mp3 catalogs. iTunes libraries are maintained at %My documents%\My Music\iTunes. The MP3s themselves only exist in one place but both MC and iTunes have their own views to them. This has worked for me for over a year.
More importantly than "let iTunes manage my library" is to uncheck "Copy files to iTunes Music folder when adding to library" as I wrote in my last post. This will keep iTunes from duplicating the MP3s as it adds them to the library. This may be desirable under some circumstances, but I can't figure out what those circumcstances would be.
Further, as you add MP3s (either via iTunes or MC) you can simply point the other application to the same folder(s) on disk and it will update its library. Both MC and iTunes are smart enough not to import files already in their respective libraries.
What might be a bit confusing is the concept of "library." It's important, I think, to mentally uncouple the MP3 files on your hard drive(s) from the lists that MC and iTunes build from those files. When you're importing/exporting you're reading/writing the internal lists, not physical MP3 files.
I'm not too familiar with the iTunes Import functions, but they appear to have more to do with reading a text file of some sort that already knows where files are (for instance, an XML file previously exported from iTunes) and would be useful if you're transfering your library to a different computer (and want to keep the extra-MP3 metadata, e.g. play count, last played, etc.) intact. Both MC and iTunes can export their libraries into XML files but they are incompatible with one another.
You can export playlists from MC (as m3u files) and iTunes can Import them, but Smartlists are "flattened" into whatever files they contain at the time of export. (MC and iTunes have completely different, proprietary methods of creating automatic playlists. MC is far more powerful but iTunes does everything that probably 85% of all users need, and have the mental capacity for.)
MC's other playlist export format are interoperability with other MP3 jukeboxes, or for display (the HTML output for instance, could easily be used on a web site). As before, the "smarts" behind smartlists aren't exported; only the song list generated at the time of export.
In summary, in iTunes, uncheck "Let iTunes manage..." and "Copy files to iTunes..." Point iTunes (as per my previous post) to your MP3 files and let it build it's list/library from the same set of files that MC is using. I hope I've done more good than harm (in terms of confusion) with this post but if you have any more questions don't hesitate to shout back.