After a long absence, I can report back to say I had no success converting my Vivitar 100mm Macro from C/Y to PK. Unlike what happened to nparsons13, in my case an original PK mount did not fit the Vivitar perfectly -- it was a little bit too small. I spent an afternoon cutting hard plastic trying to make a flat O-ring which I could screw onto the Vivitar's back and then screw the PK mount on top of it, since the screws would not be in each other's way thanks to their being misaligned -- but that didn't work either. The good news is I could find a PK-mount Cosina 100m Macro with AF for under USD 50, so I'll probably be selling or giving away the Vivitar in the near future.

Anyway, thanks for everyone's contributions -- they were much appreciated, even if I couldn't convert the lens.