Ferrets are carnivores which means they only eat meat and therefore they should not eat bananas at all. It is actually advised against by veterinarians because bananas have sugar and therefore ferrets should not eat fruits at all.
Even though your ferret may hold a soft spot in their heart for sweets and sugars do not feed any to them as they are bad for their health.
If your ferret was to eat bananas it could cause them to get insulinoma. They can also cause a very dangerous adrenal infection which can be very costly to treat and require an operation.
Ferrets also have a very sensitive system and they have difficult digesting vegetables and fruits. They are solely carnivores.
Therefore no do not feet bananas at all to your ferret as it will make them sick and possibly cause major health issues for them.