Examine these subjects
Not sure I understand you?
Nearly every option in Blender can be toggled as a key in animation.
If you need IK and then suddenly Fk.
- Develop both rigs (IK, FK)
- At a certain keyframe disable and enable rigs.
But my choice would be to have animation sequences done in IK and then in FK.
In Blender video merge image sequences.
But I think you are thinking of using them in a game engine.
problem is the same, solution too, exporting two animation sequences.