First I would create a .chd file of the appropriate size using chdman. Next I would add the files to the chd using imgtool. ex:
chdman -createblankhd blank.chd 306 8 17
(This creates a 25 MB image file I think)
imgtool put pc_chd blank.chd <nameofyoursourcefile> <nameofthedestinationfile>
(This "puts" files into the image one at a time)
I may have ommitted something here because I am in hurry to get back to the phone (it rang while I was typing this!)
If I can be of more help let me know.