As far as I know it is only possible to sync the entire vault and inside it exclude some folders.
In your case you might want to create a new vault that just contains the stuff you want to have synced and than link to the documents from the new vault into the main vault by change all the links. If you have absolute link path a find and replace would allow easy change of the path from the old folder into the new vault.
The only other option would be to sync the whole main vault and exclude all other folders in it. But this would prevent you from syncing the main vault with another computer later as you can only have on sync per vault set up.