Richard MacManus has a guide to moving comments to PyCS, although he ended up doing it in a really difficult way, so I wouldn't recommend that anybody follow his instructions!
Try this instead:
1. Fill out the form.
2. Ask me to import your comments.
3. Open Radio and go to the comments preferences page.
4. Uncheck the box that says "Check this box to use comments".
5. Edit your templates and put in the bits that the form (see #1) tells you about. Put the <script src> stuff in your home page template and the comment link stuff in your item template. Put it after the <%commentLink%> and <%trackbackLink%> bits. Radio needs those if you want to go back to using the UserLand server later on.
6. Re-render everything - right click on the Radio icon in the system tray and select "Open Radio". Now click Radio > Publish > Entire Website and wait for a bit.
Now your comments should be working ;)
This way doesn't require you to edit any of Radio's source code, and won't screw up in future if the Radio developers decide to change some of the code you would have changed. My apologies to Richard, who ended up doing way more work than necessary because I didn't figure out this method in time.