Further to my original question, the guys at dBpoweramp responded stating that the Apple Lossless Converter Codec was reverse enginereed as Apple do not provide any documentation or support to 3rd party developers. Their suggestion was to convert all my FLAC files to WMA Lossless and then using iTunes, import them using Apple Lossless.
This worked fine – although after having imported everything (yes I know I should have run a batch test 🙂 ) I noticed that all my artwork was missing. Researching this further this was because I had originally used the RipStation software which doesn’t embed the artwork but instead saves it as ‘folder.jpg’ file within each and every folder.
Thinking that I was going to either have to re-rip or at least go through and manually tag everything – I was told by the dBpoweramp guys to use their ID Tag Update Codec which will update/embed the artwork in batch mode.
So – right now I think I have the problem solved – I’ll only know when everything has been moved across to the NAS. Anyway – just thought you might like to know.