I would export the HDV timeline and then lay it onto a DV timeline and export again. That is what I do most of the time because I like to have a timeline for each. I like to keep my HD timeline in tact too. Plus, you will have problems because of the aspect ratio. If you are not careful, your exports aspect ratio will be off.
The other was is to export using quicktime conversion and find a DV preset (there is one for DVD i think) and make it letterbox to keep aspect ratio. It's under the options. Hope this helps!