You can do it in VS Code. I wouldn’t say it’s simple, but doable.
Please, test this before using it on actual notes, and don’t forget to backup.
Find (in regex mode): ^(?<![\s\S\r])
Replace: ---\nkey1: value1\nkey2: value2\n---\n\n
Edit keys and values according to your needs, separating each line with \n.