I hadn't thought of creating new fields. I'll have to test that.
There is an option that I used, based on a forum thread, which is to export your main Library as an XML Playlist, then import the file into Media Center. However, you will need to modify the XML file so that JRiver will be able to locate your files. Use the Find/Replace feature in the Mac program Dashcode (it comes with the OS; look under the root directory in Developers/Applications or search Spotlight for "Dashcode").
Open the XML file in Dashcode, and modify the directory location of each of your files.
**I would need to know if you have your music library on your main hard drive or on another drive (like an external drive) to tell you what to replace the directory info with.
TIP: Before trying this on the main library, try making a test Playlist (or export a present one) of around 20 songs. Export as XML and test it using the directory modification. Once your import into Media Center is successful, you'll be able to handle your main library and get it imported with info and artwork included (I noticed that I need to redirect the Comments, but that's another story).
EXAMPLE: In my case, as my Library is on an external drive, I replaced the directory header for each file. For example, the original directory header for iTunes was, on one song example:
<key>Location</key><string>file://localhost/Volumes/Tom's%20Media%20Library/A-Ha/Hunting%20High%20and%20Low/01%20Take%20On%20Me.mp3</string>
I took out file://localhost and replaced it with, well, nothing. I told Find/Replace to replace it with a blank entry so that it would cut out file://localhost.
The end result was this:
<key>Location</key><string>/Volumes/Tom's%20Media%20Library/A-Ha/Hunting%20High%20and%20Low/01%20Take%20On%20Me.mp3</string>
I used Find/Replace to find and Replace All the entries in the XML file. With a large library, I don't know how long it takes to replace them all, but it should not take long. The procedure above does not affect your iTunes library as it is merely telling Media Center where the playable files are. Any questions you have I would be happy to answer them.
-- Tom
P.S. I also have a large library of about 28,000 songs but am still working on importing since I am not done editing in iTunes.