Like I said, as far as I know VobBlanker is the only program that can add chapters without taking apart the DVD. Since I make chapters while creating the DVD the first time, I haven't had to do what you're trying to do so I can provide no further help.
I know that TMPG Authoring Works will accept VOB input and has a free trial, so you might want to check it out:
http://www.videohelp.com/tools/Tmpgenc_Authoring_Works
Maybe someone else has a better solution. Good luck.