Just dump them in a "raw" format (can use HDD Raw Copy Tool from hddguru) and zip or rar that up and leave the eventual chd step to the dev who adds it to mame.
Generally
USB->PATA/SATA adapter ThumbScrew program enabled making all USB read only HDDRawCopy with administrator rights to rip the HD
Select the SOURCE disk.
Click on the "Continue" button
Select the TARGET
Double click on the entry called: FILE
Select a location and a filename.
Make sure "Save as type" is set to: Raw image (dd image)
Click on the "Continue" button
Verify that all parameters are as you expected.
Click on the "START" button
After saving the data, eject the disk. (*** important ***)