There is already a feature request for that but it only has 4 votes:
Vote for it if you think it is important! I suspect that the developers don’t have time to read every post…
PS. I use Sound Forge to convert m4a files to wav/flac but I agree that I would not expect the lack of support for such a common file format in a program that is sold for 500€. Unfortunately there are other problems that are even more serious (see most voted feature requests).