"CDA" is kind of a false file that MS shows instead of actual audio tracks on the CD. Redbook CD audio discs have no real "filesystem." Consider those more like shortcut files that point to an executable or a data file associated with an executable. There are no CD audio files on the disc and you would need an app that accesses it through the digital audio, ASPI, or lower levels to do a proper conversion of the waveform audio.
Of course, this is exactly what any CD ripper does.
If you copied a bunch of CDA "files" and now want to convert them after-the-fact, you can't. You still need the disc.
*NOTE:* It's entirely possible that some CD rippers use a CDA file extension also, but it's still just a poorly-named dump.