How to prevent mp3tag web source from overwriting certain existing tags?

Mac
1

So here is my standard workflow:
I usually encode wav. or .flac files to .m4a with Apple's build in AAC encoder within macOS. This way it creates two tags: ITUNNORM and ITUNSMPB. After that I use Discogs as the main tag source to tag the rest. However, this way, it overwrites ITUNNORM and ITUNSMPB, which I want to keep. So how do I prevent mp3tag from overwriting these specific tags? Thanks!

Edit: It seems I found the required option, which is the "merge" option.

However, does anyone know how to recalculate the missing ITUNNORM and ITUNSMPB now, preferably using the already encoded audio files? If possible as a terminal command?

2

Do you use "Mp3tag for Mac" or for Windows?

3

Everything on Mac, including audio encoding

4

So please use the matching support category:

I have changed it from "Support (for Windows)" to "Mac" (English) for you.