Yes, it is totally fine. Restic will still only back up what has changed/been added since the last backup run.
When you do add folders to your backup set, restic will however not be able to identify which parent snapshot to use. This is not a problem but it means restic has to scan all the files to detect if they have changed or not, so it’s just a matter of more disk I/O on that first backup run after adding more folders. Subsequent backup runs after that will use a parent snapshot automatically and not have to do the extra scanning.
If you add folders and want to avoid the extra scanning, you can use the --parent option to the backup command (giving it the ID of the last backup snapshot before you added folders).