From the WhatsApp FAQ it looks like it is unfortunately not possible.
Also there is info on the website saying:
Your WhatsApp messages are automatically backed up and saved on your phone's memory daily.
So that must mean that it is saved in the application sandbox on iOS - therefore there would have to be an in-app mechanism to import the data, which seems to be lacking as the only feature close to this matter is Archive All Conversations - one way only.
The previous FAQ link expired, but the information still holds on a new FAQ page:
Move to a different type of phone
You can’t transfer your chat history