You would need to know what format the .joined file is in. Perhaps it’s an ISO image, in which case renaming it .iso would allow you to burn it with DropDMG.
Secondly, I think DropDMG is a program that works only with Mac computer, mine is a PC, and I have Windows XP. But I try to rename the joined file with the extension *.iso as you said, but no program want to read it as a image file. So… any other suggestions ?