I was hoping to do the same thing but it looks like you can only transcode after ingest. I've done this with my DVCProHD footage using Compressor to convert to ProRes and its worked great. Just make sure your source settings match your output settings.
The option is to edit natively and convert on output.
OS 10.5.5, Mac Pro 2 x 3 ghz quad-core intel xenon, 9 gb ram, with BM Intensity Pro card