FL Studio has a plugin called Slicex, which makes that task easy.
Load Slicex as a new channel, by going to Channels->Add One in the main menu, and choosing Slicex.
Find a vocal track to slice, in wav format. Drag and drop the sample into the waveform area towards the bottom of the plugin window.
Slices will automatically cut up the sample into a lot of pieces, and assign each piece to a different piano key.
You can then open either a piano roll, or step sequencer, and paint the voice fragments like you would notes.