Yes, there are a Winbox Add On Manager but... its 126.96.36.199 versión, too old. We need the new versión, almost 3.18 because it fix a big vulnerability in winbox 2.9.
Who can add it to Addons?
See my reponse here
Although our various support queues will be monitored for emergencies, Devolutions' offices will be closed on June 24 and July 1st, 2019.