Archiving Time Machine backup to a Synology NAS

I don't believe this is going to be possible.

The Synology is a network file server aka NAS and such devices store Time Machine backups inside a 'sparse bundle disk image'. Local backups like your USB drive are stored directly as files on the backup drive.

It is possible to move backups from one local drive to another, and it might be possible to move a network Time Machine backup from one server to another but I cannot see a realistic way to convert from one format of Time Machine backup to another.

As I mentioned local drives i.e. USB use a different format for Time Machine to network servers e.g. Synology. Local drives use Apple's HFS+ format - they will not even work with Apple's newer APFS disk format.

Time Machine relies on a HFS+ specific feature which is to create 'hard links' between copies of the files, this allows a single copy to be present in each generation of the Time Machine backup and not take up additional storage each time.

Apple get round the fact network servers do not use HFS+ by creating a 'sparse bundle disk image' and the disk image acts as a virtual HFS+ formatted drive.

You could in theory either manually create or re-use an existing sparse bindle disk image and 'copy' the contents of the USB drive in to it. In reality this as far as I know has never been done, is likely to have problems due to the complex way Time Machine works and take an extraordinarily long time - with no guarantee it would work. Assuming it did work you could then 'mount' the image and manually tell Time Machine to 'browse' this non-standard location.

You could try simplifying this by making a disk image back of the USB drive.

Backing-up your backups:

It’s generally not a good idea to copy or back-up your backups. If there’s a problem on your primary backups, it will be copied to (and perhaps magnified on) the secondary ones.

If your backups are on a Time Capsule, you can "archive" them to a USB drive connected to the Time Capsule. See How do I back-up my Time Capsule and/or Air Disk? As you’ll see, there are some downsides, however, so that's probably not a good strategy.

from this article by the late James Pond, in which he discusses way to proceed when you want to backup your backups:

.Pondini's Time Machine - Frequently Asked Questions -- 27. Can I have "dual" backups? Or back up my Backups?

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There's no way to do that. You'll have to stick with the new backups on the NAS.