I checked the limits in Poser and they were there (min. about -100000). Well, I loaded D3 (that would be David, Miss Nancy ;o) in Poser, injected all Head and Body morps and saved him as a new figure in my runtime.
Then I switched to C6 and loaded this new figure. 
Believe it or not, but this time the negative values have been accepted. Hmm, I don't know why it is that way, but I'm totally happy now that I found a way for me to make it work.

Thank you all very much for your help!

Take care and have a good day,

~Sassy~