With those particular requirements, at present, there is not a way.
If the clients are all Flash, it can be done with cooperation of the Flash client using NetStream.onPlayStatus event handler and “Netstream.Play.Complete” notification to start playing the live stream when the pre-roll stream is done.
If the client is not Flash, there is not a way to do vod pre-roll for live stream at present. There is a work-around for iOS:
A similar work-around could be done in a Silverlight player. There is not a similar work-around for RTSP devices and players.
You can play intermittent advertising in a live stream using the Stream class. You can use the scheduler example:
Using this method, the stream can be played on any client and protocol that Wowza supports. But the ad will be intermittent, like tv, not a pre-roll, so it does not meet your requirement.