While there might be some super easy method I don't know about, but these are my thoughts on the matter. Personally I have iTunes set to NOT manage my music collection. I then put my video files where I want them and add them to the library. They're all in the places I want. If I rip a CD or buy a track from the iTunes store, it will put them in the usual 'managed' locations you'd expect if it WERE managed (namely where your library is set to be inside whatever subfolders iTunes would normally use/reorganize to). That's fine if you're starting from scratch, but you seem to already have them in your library.
What I would do in that case is select within iTunes all the video files you want to move and open a finder window (or Explorer in Windows). You would then drag the selected files right into the finder location you want to move them to on the external drive. iTunes will COPY all the files there (this is also handy when making up things like USB memory sticks for newer car stereos, although OSX is bad there because it adds invisible Unix files that trip up my car stereo so I use my XP machine to load only the actual files onto the USB stick). Alternatively, you can go into your library and select all the files manually from there (you'll probably have to do that later anyway to delete the originals; but then you'll still have to reset your library so really there's no difference in the end). So you now have a copy of all your video files on your external drive. If you now delete the originals and try to play one of them, iTunes will notice it's missing and ask you to point it to the correct location. That's fine for a few files, but for a lot it's probably better to just DELETE all the video files from your library (if you selected them earlier to move them you can do this easily by hitting delete while they're still selected) and then ADD them back from the external location (make sure iTunes preferences are set to NOT manage your library locations or it'll move them right back again!) Now they'll point to the right place. You might lose things like adjusted volume settings and what not that aren't stored in the actual files themselves (not tagged on the file), but otherwise, they should line right back up.
Note that I haven't actually tried moving a large group of them this way, but have done most of the operations indicated on small numbers of files in various operations.