Problem solved, at least for Poser 7/8.

I called a friend of mine and he did some tests.

The "Smoothing Group ID" is just a number to identyfy a seperate group. You can not define a certain "crease angle" for one of this group.

What you can: Define a crease angle to a prop/actor "exept for one or more groups". The exepted groups are set to 0. Sharp edges (based on normals) are defined for each defined group.

Load a sphere as a prop.
Open the group-editor.
The whole sphere is defined as one group.
Now create an empty new group and select it.
Mark some facettes to become part of the new group.
Give this group a "smothing group ID".
Repeat to get another group with another smoth-group-ID.
Close the group-editor.

The preview will show you the groups without the common ugly black-shading.

That means:

It is possible to create "sharp edges" directly from within Poser (tested with 7/8). This is good for simple props.

Or we can create obj-files with each modeler able to define standard-groups. A script like the one I posted already can change defined groups to entries Poser is able to use for (not) smoothing.

Extend the script with 

...
  if l[0]=="g" :
    if l[1].startswith("groupID_"):
      groupid=l[1].split("_")[1]
      print >> fhout, "s %d" % int(groupid)
...

With your modeller create some groups with name "groupID_1", "groupID_2", "groupID_nnn".  
With a modified script it's also possible to use material-zones insteed of groups.

Sorry, can't explain it better because i'm not good with the english language.

---