You can do that by extending the length of your sequence so it plays past the midi notes and records the tails of the audio.
Then you can set the length of the sequence back to your loop length. Play back the sample polyphonically (duo-phonic is all you need depending on the tail length) in sampler mode.
This will let the tail continue to play as the loop starts over and plays the beginning of the sample mixed with the tail.
It would need to be done in sampler mode so you lose the ability to time stretch the loop. Not a problem if you are okay with a set BPM.
You could also use two of the same sample on 2 separate pads, double the sequence length, and alternate between them. This would obviously cut into the 16 available pads per project, but would give you time stretching capabilities.