Hi! I assume you’re talking about crossfading between several mixes which should stay strictly synchronous, by the use of a continuous parameter automation, with seek speed enabled.
What you describe is indeed a missing functionality of FMOD: the possibility to add a seek speed on volume (and other attributes). That way, you’ll be able to use a discrete parameter, or continuous without seek speed on it (it will react instantly, going for instance from 4 to 2, as you want), but the cross fade would still be possible by that volume seek speed.
The more we tell FMOD staff this feature would be great (which I really think), the more are the chances they implement it soon 
However there’s some (a bit cumbersome) workarounds. You can read the answer I gave on that thread, it may give you some leads.