OK, then in that case I assume you already use LR "Classic" as your main (synced) catalogue. It only uploads smart previews which are 2560 pixel images (longest size).
Identify these best images in Classic, sync them (eg via a Best Photos collection), then export them and choose the Original format in the Export dialog. Open LRCC and import these copies which will then be uploaded to Adobe's cloud. Select an image in LRCC and the Info panel (bottom right) will show that you have an original in the cloud.
Adobe's server then detects that the newly-uploaded original matches an image that is already synced, so Classic won't download it again. You're now left with your original catalogued in Classic, and a copy original in the cloud.
I hope I have all the details right, but it's not meant to be easy or slick. Adobe see you using either LR Classic or LRCC, not both. Even if I do understand what's happening, I think I'd find it too confusing.
Maybe you would be better off simply storing copies of the originals in the cloud storage folder that comes from the CC App (see its Assets tab). This is more like a Dropbox folder, and I think the 20gb covers both this space and the synced assets.
John