The easiest way to do this is first sample in pattern or song mode using slice with seq.

This will create a walk you through the steps of creating a Sample Voice containing a waveform with a number of keybanks that are the individual slices. It will also create a MIDI file to drive the slices. So Motif automates most of this process.

Using Sample edit, take a look at the name of the waveform that was created in the Sample Voice - User001 for example. When you want to create your User Voice. Go to Voice Mode, initialize a user voice, press edit and then the 1 button to select the parameters for element one. Press F1 [osc]-SF1 [wave] and select Use as the wave nak and then the number of the user waveform for sampled voice ( in our case user001). Now store this as a user voice.

Go back to pattern and select mix and in F3 voice replace the Sample Voice automatically created for track by Motif with the User Voice you created.

Store your mix and save your pattern. If you want to use that sample voice in another song or pattern, just call up the User Voice in a Mix.