Solution
I asked the support and here is the working solution:
1. Open File -> New
2. Choose "New installation from DVD or image file"
3. Proceed with creating a virtual machine and please choose 'Continue without disk'
4. Check "Customize settings before installation" and continue 5. In virtual machine Configuration window click Hardware tab -> Hard Disk1 and from the 'Source' list choose Boot Camp partition
For more details and a German Article about this look here: Link
I hope that will help all users, who have problems with this! 
Edit:
I would be very happy, if you could gave me a Feedback for the short Instruction above. Maybe rate the Blog-Post or post your Experience with this Solution. Thank you!
Last edited: Oct 18, 2012