This feature is only available in SoundPrism Pro - get it here:
SoundPrism Pro can also be triggered from other Apps or MIDI-Controllers using Core MIDI via WiFi or USB (iPad only). Of course this also means that SoundPrism Pro is background audio capable.
Since SoundPrism can be triggered externally, arpeggiators like StepPolyArp can be used to trigger SoundPrisms sound engine while they're being fed MIDI notes from SoundPrism. Here's a video explaining this in detail.
Finally, it's even possible to turn off SoundPrism Pro's reaction to its own MIDI Notes in this setup so it only plays arpeggiated patterns.