Your PROD code should be in the main branch. Then create and checkout a new branch from main, let's say import/UAT.
On the import/UAT branch, do a file diff , not a git diff against the code in your existing UAT branch. You may need to copy those files to another location to do the file comparison. I'd use something like Beyond Compare. You can copy/commit those changes into import/UAT.
Now you'll have your git history between the two environments.
While on import/UAT you can do a git diff main --name-only to see a list of file names that have changed and work from there.
Update:
If you do a file compare with a good tool, you can bulk copy all changes from the source UAT folder to the folder containing your repo on import/UAT. Then you can bulk add all changes in one command.
git add .
git commit -m "UAT changes."
You can even try just copying the existing UAT files into the repo folder while you have import/UAT checked out. git will most likely figure out file deltas and additions regardless of the file's timestamp.