Bryan Creer had a converter program to get ABC into Noteworthy and vice versa - everyone I've heard of who's used it has liked it. That has to be better than detouring via MIDI.

Abc2Nwc is available here and yes indeed, it's very good.

It doesn't work with newer v2.0 NoteWorthy files though; so if you want to move a tune from NWC 2.0 to abc, you have to open the file in NWC 2.0 and export it as an NWC 1.75 version file first.