Why don’t you just map the sample to the same key in two keygroups, assign the modulation wheel or even a Macro switch to osc gain, one of them inverted and then use the wheel/switch/controller to crossfade/switch between the two. Using non-retriggering LFOs or multi envelopes assigned to grain position in the granular oscillator is the closest to what you want to achieve but once triggered they never reset until the release phase of the ADSR is over.
