It is possible, but you have to modify generated code.
Currently existing .gmfmap editor does not allow you to specify non-containment
reference as a "containment feature" for child node reference, but you should
be able to do it using text editor..
Then you "hack" .gmfmap model this way you can ignore all warning and generate
gmfgen model /code from it. In the end you have to modify ???CreateCommand
generated for these child elements to store newly created elements in a proper
containment feature in addition to non-containment you've specified above.