The first episode of Parks and Recreation featuring Lucy as Diane is scheduled for 04 October. We know that Lucy will be in a couple of episodes. The second episode will screen 25 October 2012. Here is the info about the episode:

Season 5 Episode 5: Halloween Surprise
Airdate: 25 October 2012

Ben gets a job offer, forcing Leslie to think about their future; Ron and Andy take Diane's girls trick-or-treating; Tom looks for a new business idea

For more about Lucy's role as Diane on Parks and Recreation Season 5 visit the AUSXIP Lucy Lawless Parks and Recreation subsite