You can use "dd" to read raw blocks off of a hard drive. Most emulators aren't set up to handle partitioned images though, so to create .PO images for an emulator you'd have to extract the partitions. You might be able to accomplish this with the HFS driver.
KEGS makes excellent use of partitioned devices in Linux. This includes HFS formatted volumes. Yes, cdrom's too. Just be careful and not have them mounted in KEGS and also opened in another Linux program with write access, to avoid possible data loss or corruption.
Cheers, Mike T.