If this is your own voicebank, you can improve the tone of longer notes by recording longer samples. This will give the resampler more to work with, so that it's not stretching out a tiny amount of sound across a much longer timespan.
You can also try using a looping resampler, such as tn_fnds or Moresampler. Rather than stretching the audio, it will loop back and forth to extend the length of the audio. Looping resamplers are particularly useful for noisier vocal styles, such as whispering or screaming.