Reverse IK // change pivot/root of an armature

hi

Would it be possible to make an new bone contrainst in which the pivot (root bone) point is changed ?

Example : i make the left foot the pivot point, then i make an IK starting from the hip. So i can move around the hips and the foot stays in the same place. Just like i can rotate the foot and the whole body rotate.

It has to be a constraint so i can bake the animation and i dont mess up the inner animation by modyfing the armature’s hierarchy.

How could i achieve this ?