I think its a bug.
If you try to append a pf source on a Particle Skinner modifier you get an error
_pfSource = PF_Source()
_particleSkinner = Particle_Skinner()
--try this
append _particleSkinner.Particle_Flow_Systems _pfSource
--or try that
--_particleSkinner.Particle_Flow_Systems = #(_pfSource)
Runtime Error: Invalid value for property: $PF Source 001
Any ideas?