If you are using Compressor 3, its easy.
Just drag the setting for DVCPRO HD from Advanced Format Conversions to your clip in the batch window. Then in the Geometry tab of the Inspector, select 4:3 in the Output Image Inset (padding) inset section.
I don’t think you can do it properly with earlier versions of Compressor. I used to use AE for this, but Compressor 3 does it nicely.