I would think that you could temporarily connect the USB drive and the new firewire drive to the Mac simultaneously, copy the files from one drive to the other, and set up the copied files on the firewire drive in the iTunes library.
What might be behind your understanding that "there is no way to use those files in iTunes" (on the Mac) is that if the USB drive is formatted with the NTFS file system (as opposed to FAT32), Mac's wouldn't be able to write to that drive (without using certain third party software programs). However, as I understand it Mac's are completely able to read files from NTFS-formatted drives, which would allow you to copy them to the new drive using your Mac.
Regards,
-- Al