unless you get seed you wont be able to get plants from the flowers. But spring would be a good time for taking cuttings.
welcoe to GoY too.

My Fatsia Japonica varigata are looking superb at the moment....don't think ive ever seen so many flowers

trouble is there isnt much about to polinate flowers this time of year so if you want seed youl have to self polynate . then you got to hope the first frost doesnt kill the flowers of .cuttings as said are bye far the best idea .