HDtracks downloaded files are not proprietary and are common industry standard AIFF, ALAC, FLAC or WAV files, depending which you selected to download (but you obviously didn't download FLAC as they cannot be natively played in iTunes).
Audirvana Plus 2.0 supports those file types as well as DSF, DSDIFF and SACD ISO.
If you are subscribed to iTunes Match ($24.99/year) that will let you store all of your library including CD rips or downloaded files purchased elsewhere.
Steve