The manual mentions you can play remaining voices in sequencer mode but I just found there is some exploitable behaviour in the arpeggiator too.
If you engage hold whilst the arp is on, each new keypress replaces the previously entered notes. However, if you hold a chord using the hold button and then turn the arpeggiator on, you can add notes to the arp pattern. You can then vary these notes or add others (played at the same time) by continuing to play.
If you play one of the previously held notes they are all still held in the arp but once you play one more they start getting subtracted.
Try it n let me know if I have it correct