The runtime will use and interpret whatever is stored in the exported skeleton.
If the exported skeleton has IK data, the runtime will use that information to do IK.
Are you asking if you can just add IK at runtime if the exported skeleton doesn't have IK data?
I suppose you theoretically could. It's not straightforward though (and varies between languages), since the runtime isn't designed with that in mind.
But the Spine runtimes are all source-available so it's merely the difference between it coming from serialized data, or from whatever scheme you might imagine.
You could also edit the exported skeleton json to add IK data there.
Either choice wouldn't be very animator-friendly though.