sorry for the delay to respond to your query but other commitments have seen my attentions focused elsewhere until now. With regards to adding another box yeah, sure, its certainly the time of the year that beekeepers need to be interpreting what their bees are telling them and where they are drawing comb on the top of existing frames understand that the the hive is telling us. That said the addition of another box on top of an existing hive wont prevent a hive from swarming as if they as a super colony have decided that the season is looking very prosperous and that they are able to manage the risk then adding another box wont prevent your bees from swarming.
Now an inspection of the hive and manipulation of frames to create more space will certainly help you manage your bees and the best in-depth guide I can provide you on this can be found at:
No this is a hell of a read but it does also provide plenty of great images of what your typically expect to see and also a few greater charts explaining how the sudden availability and influx of good quality pollen is the driving force behind the momentum of your colony.
So, the best thing you have done is to understand that your bees are telling you that there is a honey flow on where you are and that they need room, you now need to take the next step and inspect the hive to see how the colony is travelling now that we are headed towards Spring and that can occur any day with a temp of 16C or higher without too many issues.
Trust that this provides you with some guidance,