You don't. There are the 26 preset patterns, plus the random pattern sequencing. That's not to say that you can't 'edit' the patterns using the 'groove' and 'effects' sections, but those kind of tactics only selectively allow the preset pitches of a given pitch sequence through.
There has been a few posts here calling for a 'user-defined' pattern, or non-specific 'more patterns'. It's difficult to say how that could be implemented. Maybe a single, user-definable pattern, and external editing with a more sophisticated patch librarian. Or a direct upload to the device via a Standard MIDI File.
Do you use MIDI in your bass rig? I've been using DAW and hardware sequencers (as well as LFOs and other MIDI controller sources, with upper + lower limits) to emulate HarPeggiator patterns. They target pitch-shifting algorithms other than the HarPeggiator, because the 'patterns' are all generated externally.
If you're interested (and you utilize the MIDI jacks in your system), I can go into more detail about the process.