Lightroom CC: Selective Smart Preview

Are you talking about Lightroom CC for Mobile? You can add that current work to an album, and then mark this album for offline editing.

I think I knew about the option in the mobile app. But I'm thinking more about the mac os version

I don't think there's an option to "keep Smart Previews locally for selected files ONLY", which I think is what you're asking for. However, with judicious use of the controls on the Preferences>Local Storage tab I would think that you'd be able create the scenario you want as I would expect LR to prioritise the retention of recent imports over rarely accessed files. Specifically, keep the "Store a copy of all originals locally" unchecked, keep the "Store a copy of all Smart Previews locally" checked, then play around with the percentage amount of free disk space on the system drive for the photo cache size. Keeping the percentage amount low should force LR to "manage" the amount of space it's allowed to use, so that's where any smarts that it has should come in to play.