Right now it isn't possible with the 16 levels function. You can can adjust the tuning modifiers after you have recorded your notes though. I do this sometimes, just record a bunch of notes to get the rhythm I want. Then go into the automation lane and adjust the tuning modifier with the mouse to get a tuned rhythm going. It's not the greatest thing but you can get the same result in theory that you're trying to achieve, you just can't play it unfortunately this way...
Here is a way you can do it that I just thought of while writing what I wrote above.
Use Pad copy!
So make a new program and load the sample that you want to do your custom tuning with.
Use Pad copy and duplicate the sound across all 16 pads. Then on each pad go into program edit and adjust the tuning of each pad to your liking. Now you have a custom tuned 16 levels program of that sound.
You can do more and apply different filters and FX to each pad as well to really get dynamic with it.