Do you know of a "simple" way to convert an existing keyboard app in IOS10 to be also an iMessage App? I have a containing app and an extension keyboard as such it is perfect for iMEssage.
I'm not sure about how simple, but creating a new target for you app, to have an iMessage App Extension will be a good start. There's no one-to-one mapping, but it should be straightforward.
Difficult to say for sure without knowing more about the code, or what the keyboard app does. Check out this tutorial to get started..