That would be a cool & very useful feature.  My speculation has been that the preset patterns were 'stuffed' into individual bytes of memory for efficiency.  Perhaps the overhead of having each of the 16 steps programmable was a design decision that hit the cutting room floor.

Actually, there is, but it requires sequences of MIDI CC values from an external source.  Each interval of the HarModulator's six octaves responds to a narrow slice of the 0-127 range.  Stack up the correct values in tempo-sync'ed sequence for a custom "HarPeggiator" pattern.

When using the H910/H949 algorithm [Scale/Speed: Normal] as a CC target, it's possible to program 'glides' into individual steps (found in many of the 26 preset HarPeggiator pitch sequences).  Of course, these workarounds won't have the effect sequences, but the rhythm / groove and dynamics are 'programmable' as part of the MIDI CC sequence itself.