A Numbers generator has, in most cases, been a convenience for me. You can use any compatible parameter as a source for a Slider rig. When I get involved with complex situations, Numbers allows me to watch multiple values as they change (the inspector is more difficult to work with.) Numbers is "light weight" and doesn't drag down animations — and like any other null object, you simply turn off their visibilities.
Once you keyframe a parameter, you've basically locked in its property animation. It IS possible to Rig a keyframed animation, but you will have to re-keyframe "stages" that correspond to the Rig levels (aka "snapshots")... doable... not something I'd recommend (or, at least, not an approach I would care to take — so FWIW, my response is somewhat biased).
There are ways to *ADD* onto, or multiply the object's properties without affecting the original keyframed animation (similar to the way I outlined above). The Custom behavior is always *additive* so it can be added to a keyframed parameter. Other behaviors can be additive or multiplicative... you just cannot divide by a variable amount — you have to be able to set up a multiply by decimal amount. [Link has options to Replace, Add or Multiply values.]
For instance, you could duplicate the object you keyframed (whatever properties you used) and clear the keyframes from it. Then, Link the same parameters together. If you publish the Custom Mix parameter of the one you Linked, then the keyframe animation would be "combined" (from the default you set of the un-keyframed object to the keyframed object). Which object you prefer to "show" is up to you and depending on how you set up the custom mix to act. One way would have the default apply to the animated (to which Link is applied) and the other would be to have the animation to apply to default (to which would have the Link). You wouldn't (and couldn't) Link both objects at the same parameter (it would defeat the purpose and very likely crash Motion,...LOL). In the first case you would show the animated object — in the second case you would show the default object.
As an example: below the square is animated from the left to the right. The circle is Linked to the animation of the X Position of the square. The Custom Mix is "user set" at 0.5 (50%), so the animation is halved. The user can set any amount of that animation with the Custom Mix — which can be applied to a Slider Rig if you prefer to display the amount of animation in the Range of 0 - 100. In this case, the circle would be shown and the square hidden (either by turning off its visibility, setting its opacity to 0 or moving it off the canvas.)
This only begins to scratch the surface of what can be done in Motion...
