As I understand it, Noteworthy Composer can hold files in a compressed format that they do not publish the specification for, so there are few if any other programs that can read them. You can save Noteworthy files in an uncompressed format that quite a lot of programs can read, including some for the Mac. So much depends on how exactly the 'many, many' files are currently stored. Your first step might have to be to convert them all to an uncompresse format.

Anyone who actually owns Noteworthy Composer can probably correct my understanding!