I don't think a dynamic hotspot is the right approach. What you probably want is some sort of custom movement, like the move safely .cap that was made a while ago. After colliding with a sphere (or getting close to one, set a variable in the sphere, then move the player towards it. Even better, if you're using the platform movement, you could get the x/y speed of the platform movement, deactivate it, activate the custom movement and put the speed values into it for a smooth transition.
(I haven't used the custom movement behavior much at all, so this might not work, but from the cap I tried, it seems like it should.)
Edit: Here's the cap.