Can humans use god ki?
Simply yes, not only humans, all mortals could obtain or use god ki via various ways:


Through special rituals, such as Super Saiyan God Ritual. Unfortunately this kind of rituals don't exist throughout human history, but maybe it do exist in other races.
Highly intense training under the supervision of an angel, for example Whis. It's most likely to obtain god ki by entering the Hyperbolic Time Chamber.


(Source)